It's Run that actually starts a Browser, not NewContext. If the browser
is closed or crashes, the browser handler will fail to read from the
websocket, and its goroutines will stop.
However, the target handler's goroutines may not stop. The browser
handler uses a separate cancel function to stop itself when encountering
a websocket error, so that doesn't propagate to the original context
children, like the target handler.
To fix this, make it so that NewContext can keep the cancel function
around, for Run to use it in this scenario. And add a test case that
tests this very edge case, which used to time out before the fix.
Fixes#289.
For all contexts except the first browser context, as in that case the
allocator and browser handler already take care of shutting down the
process and goroutines, respectively.
Fixes#293.
Chrome already starts with a blank page, so use that for the first
target context instead of creating a new tab.
Add the first version of the Targets API, which is useful to test this
feature.
Fixes#291.
