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Implements a HitTestAggregator component to facilitate cross process
hit testing with minimal process hops.

The HitTestAggregator maintains a list of HitTestData objects that will
be created and sent with each CompositorFrame.  These are aggregated
into a single DisplayHitTest data structure that can be made available
in shared memory to facilitate hit testing with minimal process hops
for any Display.

HitTestAggregator listens as surfaces are added to the current
CompositorFrame to ensure that the list only includes information that
matches what has been rendered in the current frame.

Aggregation occurs on the BeginFrame notification.

This change includes the implementaiton of the HitTestAggregator and
unit test cases to verify the intended behaviour.

Next steps will include completion of the creation and transport of
HitTestData objects, implementation of shared memory and using this
component to target incoming events.

BUG=732399

Review-Url: https://codereview.chromium.org/2938953002
Cr-Commit-Position: refs/heads/master@{#486852}