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This introduces a new mechanism for determining when
we're ready to activate the pending tree. The tile
priority is still used to determine when it's worth
waking up the compositor thread and evaluating if
we can activate. However, the actual check that
determines if we're ready to activate doesn't rely
on the state of scheduled raster tasks but is a
synchronous call on each layer.

The result is a pending tree activation mechanism that
is much easier to debug and validate for correctness,
while still providing the performance benefits of the
old mechanism by taking the "required to activate" field
of the tile priority into account when scheduling tasks.
 
BUG=375206
TEST=cc_unittests --gtest_filter=PictureLayerImplTest.AllTilesRequiredForActivationAreReadyToDraw

Review URL: https://codereview.chromium.org/287643004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272635 0039d316-1c4b-4281-b951-d872f2087c98