Update compositor hit testing tests to read rects back from WebLayer

This enables testing a few additional scenarios by covering the code path that stores an SkRegion into the compositor.

SkRegion internally will split rects up into smaller adjacent rects in order to maintain it's sort invariant for performance.  This change merges such rects back together in the Internals API to simplify the test output (and make manual verification easier).

This change also removes the ability to track rect update counts, but this was really a perf concern not a correctness one, and so is better handled by the perf testing I'm doing.

The test output now exposes a pre-existing bug where rects from layers that have had all handlers removed are left behind.  http://crbug.com/271747 tracks fixing this.

BUG=270922

Review URL: https://chromiumcodereview.appspot.com/22260005

git-svn-id: svn://svn.chromium.org/blink/trunk@156053 bbb929c8-8fbe-4397-9dbb-9b2b20218538