Opening pages in a new tab might suffer an extra delay from e10s-multi because
the new process has to start up and then run all the process / frame scripts
before it can react on the request from the parent to load the first page.
There are two code paths. Either we start the tab with a remote browser and
then the RemoteWebNavigation will send the request. Or we start with a non-remote
browser and have to change the remoteness flag on it, and then the SessionStore
will send the request.
In each cases we start the timer on the parent side, send it with the message,
and when the child receives it it stops the timer and reports the measured delay.
The primary browser navigational UI should now behave as if it's connected to
the page content in the viewport, including things like:
* Content page's URL is displayed in location bar
* Content page's title is displayed on the tab
* Back / forward navigates the viewport
* Entering a location navigates the viewport
* Page loading progress is displayed in the status bar as usual
MozReview-Commit-ID: FzxWEwj13sJ
