For Firefox DevTools, we will need to track WebViews by a numerical ID.
Here, we just increment a static 64-bit counter. We can switch to using
IDAllocator if we ever have an issue with this.
This patch adds such an ID to the views and a couple of APIs to access
WebViews after creation.
Some tests take longer than others, and so may want to set a custom
timeout so that they pass, without increasing the timeout for all other
tests. For example, this is done in WPT.
Add an `internals.setTestTimeout(milliseconds)` method that overrides
the test runner's default timeout for the currently-run test.
We will want to re-inform WebContent of the system visibility state when
we create a new process after a crash. This changes the IPC to just send
the enum value directly, instead of a boolean, so that we can just store
that enum value directly on the ViewImplementation class.
