Track execution phase of browser process and track it as
a UMA histogram.

As a first attempt, tracked the following 

- Before starting metrics service,
- Before creating profile
- Before starting startup time bomb
- Before starting thread watcher
- Before starting main message loop
- Before starting shutdown time bomb
- clean shutdown.

R=jar@chromium.org

Review URL: https://codereview.chromium.org/28283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231334 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231334