O(n log n) sweep line algorithm for computing jank areas.

This potentially replaces JankTracker's current use of blink::Region,
which is worst-case quadratic.

Initial tests suggest the sweep line is slower than blink::Region for
simple regions, but faster for complex ones (even with low rect counts).

Landing behind a REF (JankTrackingSweepLine) pending further testing.

Bug: 581518
Change-Id: I0b0ee613acb265576791661ac3af484287421111
Reviewed-on: https://chromium-review.googlesource.com/c/1348851
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610713}
8 files changed