[FastTextAutosizer] Add chrome://flags entry for the fast text autosizer

This patch adds a chrome://flags entry for enabling the fast text
autosizer on Android. This is a first step towards getting
some early data before enabling this on the dev channel.

This patch has added CONTENT_EXPORT to kEnableFastTextAutosizing
so the flag is accessible from chrome/browser.

For more information, see our doc: http://tinyurl.com/fasttextautosizing

BUG=342474

Review URL: https://codereview.chromium.org/146893006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250546 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250546
Add test plumbing so that layout tests can query whether a compositor frame was requested

BUG=336676

Review URL: https://codereview.chromium.org/159493002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250549 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250549