Statically enable TCM and get rid of compositing-mode-field-trials.

This is based on top of https://codereview.chromium.org/27197009/

TBR=joi
BUG=302738, 233830, 306557

Review URL: https://codereview.chromium.org/26389003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229806 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229806