Cherry-pick: aw: Ensure fallback tick unsets |block_invalidates_|

Clean cherry-pick of chromium

BUG: 17111986

Original description:

Currently, if |compositor_needs_continuous_invalidate_| is unset before
PostFallbackTick or FallbackTickFired, then the fallback ticks are
cancelled, but we never unset |block_invalidates_|, which will block
invalidates indefinitely.

Fix is simply always unset |block_invalidates_| even if we skip
the fallback tick.

Change-Id: I8bf65ae614a5ac697d5bc1b62c7cd11fbbd16cad
1 file changed