clean up various pieces of the API

First, collapse Browser.Start with NewBrowser. There's no reason to
split them up.

Second, unexport Browser.userDataDir, since it's only needed for a test.
It's also a bad precedent, as only the ExecAllocator will control the
user data directory.

Third, export Context.Browser, since we were already exporting
Context.Allocator.

Finally, remove the Executor interface, a duplicate of cdp.Executor.

allocate.go

@@ -142,8 +142,7 @@ func (p *ExecAllocator) Allocate(ctx context.Context) (*Browser, error) {
 	if err != nil {
 		return nil, err
 	}
-	browser.UserDataDir = dataDir
+	browser.userDataDir = dataDir
-	browser.Start(ctx)
 	return browser, nil
 }
 

allocate_test.go

@@ -30,7 +30,7 @@ func TestExecAllocator(t *testing.T) {
 		t.Fatalf("wanted %q, got %q", want, got)
 	}
 
-	tempDir := FromContext(taskCtx).browser.UserDataDir
+	tempDir := FromContext(taskCtx).Browser.userDataDir
 	pool := FromContext(taskCtx).Allocator
 
 	cancel()

browser.go

@@ -23,7 +23,7 @@ import (
 // the browser process runner, WebSocket clients, associated targets, and
 // network, page, and DOM events.
 type Browser struct {
-	UserDataDir string
+	userDataDir string
 
 	pages map[target.SessionID]*Target
 
@@ -55,9 +55,14 @@ func NewBrowser(ctx context.Context, urlstr string, opts ...BrowserOption) (*Bro
 	}
 
 	b := &Browser{
-		conn:  conn,
+		conn: conn,
+
 		pages: make(map[target.SessionID]*Target, 1024),
-		logf:  log.Printf,
+
+		cmdQueue: make(chan cmdJob),
+		qres:     make(chan *cdproto.Message),
+
+		logf: log.Printf,
 	}
 
 	// apply options
@@ -72,6 +77,7 @@ func NewBrowser(ctx context.Context, urlstr string, opts ...BrowserOption) (*Bro
 		b.errf = func(s string, v ...interface{}) { b.logf("ERROR: "+s, v...) }
 	}
 
+	go b.run(ctx)
 	return b, nil
 }
 
@@ -154,13 +160,6 @@ func (b *Browser) Execute(ctx context.Context, method string, params json.Marsha
 	return nil
 }
 
-func (b *Browser) Start(ctx context.Context) {
-	b.cmdQueue = make(chan cmdJob)
-	b.qres = make(chan *cdproto.Message)
-
-	go b.run(ctx)
-}
-
 func (b *Browser) run(ctx context.Context) {
 	defer b.conn.Close()
 

conn.go

@@ -44,7 +44,7 @@ func (c *Conn) Write(msg *cdproto.Message) error {
 	return c.WriteJSON(msg)
 }
 
-// Dial dials the specified websocket URL using gorilla/websocket.
+// DialContext dials the specified websocket URL using gorilla/websocket.
 func DialContext(ctx context.Context, urlstr string) (*Conn, error) {
 	d := &websocket.Dialer{
 		ReadBufferSize:  DefaultReadBufferSize,

context.go

@@ -2,7 +2,6 @@ package chromedp
 
 import (
 	"context"
-	"encoding/json"
 	"fmt"
 
 	"github.com/chromedp/cdproto/css"
@@ -14,16 +13,11 @@ import (
 	"github.com/chromedp/cdproto/target"
 )
 
-// Executor
-type Executor interface {
-	Execute(context.Context, string, json.Marshaler, json.Unmarshaler) error
-}
-
 // Context is attached to any context.Context which is valid for use with Run.
 type Context struct {
 	Allocator Allocator
 
-	browser *Browser
+	Browser *Browser
 
 	sessionID target.SessionID
 }
@@ -68,35 +62,35 @@ func Run(ctx context.Context, action Action) error {
 	if c == nil || c.Allocator == nil {
 		return ErrInvalidContext
 	}
-	if c.browser == nil {
+	if c.Browser == nil {
 		browser, err := c.Allocator.Allocate(ctx)
 		if err != nil {
 			return err
 		}
-		c.browser = browser
+		c.Browser = browser
 	}
 	if c.sessionID == "" {
 		if err := c.newSession(ctx); err != nil {
 			return err
 		}
 	}
-	return action.Do(ctx, c.browser.executorForTarget(ctx, c.sessionID))
+	return action.Do(ctx, c.Browser.executorForTarget(ctx, c.sessionID))
 }
 
 func (c *Context) newSession(ctx context.Context) error {
 	create := target.CreateTarget("about:blank")
-	targetID, err := create.Do(ctx, c.browser)
+	targetID, err := create.Do(ctx, c.Browser)
 	if err != nil {
 		return err
 	}
 
 	attach := target.AttachToTarget(targetID)
-	sessionID, err := attach.Do(ctx, c.browser)
+	sessionID, err := attach.Do(ctx, c.Browser)
 	if err != nil {
 		return err
 	}
 
-	target := c.browser.executorForTarget(ctx, sessionID)
+	target := c.Browser.executorForTarget(ctx, sessionID)
 
 	// enable domains
 	for _, enable := range []Action{