Send Mac software compositor swap acks immediately.

Send latency info only once the frame has been displayed. The relevant
telemetry tests (GpuTabTest.testScreenshotSync) pass, provided nothing
too stressful (e.g, Expose gestures) is done while the test is running.
This is the case before software compositing as well (it will fail if
Expose is used during the test).

BUG=286038
TBR=jbauman@chromium.org

Review URL: https://codereview.chromium.org/116393003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241432 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241432