Report page load time for startup tests.

Add the following stats to startup tests:
* foreground_tab_load_complete - time it took for the foreground tab to load
* last_tab_load_complete - time it took for the last tab to load.

All of these are in ms relative to browser main entry.

Changes in Chrome:
Report tab load time using base::Time rather than base::TimeTicks since all times are relative to browser main entry.

BUG=317481

Review URL: https://codereview.chromium.org/62833008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236009 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236009