Add viz-host HitTestQuery.

HitTestQuery finds the hit test target, for a given location, in the
AggregatedHitTestRegion list received from HitTestAggregator.

BUG=732400
TEST=viz_unittests

Review-Url: https://codereview.chromium.org/2933493003
Cr-Commit-Position: refs/heads/master@{#488854}
11 files changed