MetricsService API to support synthetic field trials

Extend the MetricsService API to register synthetic field trials, where trial groups map to Chrome configuration states, e.g., values of a preference. These trials are included in UMA reports.

BUG=270275

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=234881

Review URL: https://codereview.chromium.org/22340005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235521 0039d316-1c4b-4281-b951-d872f2087c98
cc: Mark render surfaces that ignore outside occlusion.

Generalized RenderSurface::nearest_ancestor_that_moves_pixels
to nearest_occlusion_immune_ancestor. Used this flag to mark
render surfaces that have replica, copy-request, or filters
that move pixels.

BUG=307612

Review URL: https://codereview.chromium.org/40613004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235522 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235521
Adding a flag to show all logged in users in the system tray

The multi profile feature which shows all logged in users in the system tray is moved behind an (additional) flag.

BUG=318819
TEST=unittest & visual

Review URL: https://codereview.chromium.org/59033013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235523 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235523