Warm up the GPU when the user touches the screen

Some devices perform very aggressive GPU power management. This is good
for battery life, but can lead long delays when the GPU is woken up
after having been idle. As an example, on Nexus 4 the additional delay
from spinning up the GPU is on the order of 10 ms. This can easily cause
us to miss the next vsync when responding to an input event.

This patch adds a mechanism for GPU process clients to notify ahead
of time that they are likely to draw very soon and the GPU should be
warmed up. We drive this signal with the TapDown gesture, which is sent
whenever the user starts touching the display. This fits well to the
common scrolling scenario where the TapDown is sent several tens of
milliseconds before the first ScrollBy event that actually causes
rendering.

BUG=309734

Review URL: https://codereview.chromium.org/49623002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233889 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233889
Fix flaky DeviceLocalAccountTest.ExternalData browser test

This CL fixes the flakiness in the DeviceLocalAccountTest.ExternalData
test.

BUG=316046
TEST=The test no longer flakes.

Review URL: https://codereview.chromium.org/64543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233890 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233890