GitLab

* Introduce a ProcessManagerObserver
* Make the Chrome extension RuntimeAPI implementation use it to send onStartup
  events.

BUG=354552
TEST=browser_tests *Extension*

Review URL: https://codereview.chromium.org/212603020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260478 0039d316-1c4b-4281-b951-d872f2087c98