error in Run if passed an allocator context

The correct way to use Run is after NewContext. Using NewExecAllocator
only, which also gives a context, almost worked. This confused some
users into thinking it was a bug in chromedp.

Instead, error immediately with an invalid context error.

Fixes #299.

allocate_test.go

@@ -9,13 +9,13 @@ import (
 func TestExecAllocator(t *testing.T) {
 	t.Parallel()
 
-	poolCtx, poolCancel := NewExecAllocator(context.Background(), allocOpts...)
-	defer poolCancel()
+	allocCtx, cancel := NewExecAllocator(context.Background(), allocOpts...)
+	defer cancel()
 
 	// TODO: test that multiple child contexts are run in different
 	// processes and browsers.
 
-	taskCtx, cancel := NewContext(poolCtx)
+	taskCtx, cancel := NewContext(allocCtx)
 	defer cancel()
 
 	want := "insert"
@@ -41,22 +41,36 @@ func TestExecAllocator(t *testing.T) {
 func TestExecAllocatorCancelParent(t *testing.T) {
 	t.Parallel()
 
-	poolCtx, poolCancel := NewExecAllocator(context.Background(), allocOpts...)
-	defer poolCancel()
+	allocCtx, allocCancel := NewExecAllocator(context.Background(), allocOpts...)
+	defer allocCancel()
 
 	// TODO: test that multiple child contexts are run in different
 	// processes and browsers.
 
-	taskCtx, _ := NewContext(poolCtx)
+	taskCtx, _ := NewContext(allocCtx)
 	if err := Run(taskCtx); err != nil {
 		t.Fatal(err)
 	}
 
 	// Canceling the pool context should stop all browsers too.
-	poolCancel()
+	allocCancel()
 
 	tempDir := FromContext(taskCtx).Browser.userDataDir
 	if _, err := os.Lstat(tempDir); !os.IsNotExist(err) {
 		t.Fatalf("temporary user data dir %q not deleted", tempDir)
 	}
 }
+
+func TestSkipNewContext(t *testing.T) {
+	ctx, cancel := NewExecAllocator(context.Background(), allocOpts...)
+	defer cancel()
+
+	// Using the allocator context directly (without calling NewContext)
+	// should be an immediate error.
+	err := Run(ctx, Navigate(testdataDir+"/form.html"))
+
+	want := ErrInvalidContext
+	if err != want {
+		t.Fatalf("want error to be %q, got %q", want, err)
+	}
+}

chromedp.go

@@ -154,7 +154,11 @@ func Cancel(ctx context.Context) error {
 // via one of the allocator constructors like NewExecAllocator.
 func Run(ctx context.Context, actions ...Action) error {
 	c := FromContext(ctx)
-	if c == nil || c.Allocator == nil {
+	// If c is nil, it's not a chromedp context.
+	// If c.Allocator is nil, NewContext wasn't used properly.
+	// If c.cancel is nil, Run is being called directly with an allocator
+	// context.
+	if c == nil || c.Allocator == nil || c.cancel == nil {
 		return ErrInvalidContext
 	}
 	if c.Browser == nil {