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The original change to enable scrollbar latency for the composited
scrollbars incorrectly used an existing member to try and determine
whether a GestureScrollUpdate was the first one in an injected sequence
or not. is_first_gesture_scroll_update_ was incorrect because it is only
updated when input is actually dispatched to InputHandlerProxy, and the
flag is cleared for all GSUs before the location where it was being
read.

This bug was missed because of incorrect tests. The
VerifyRecordedSamplesForHistogram method doesn't actually assert or
expect anything - the return value must be inspected.

As part of fixing up the tests, I made a few other changes to get them
passing consistently across all platforms:
- turn on main thread scrollbar injection feature (in case it's ever
  turned off we don't want the tests to start failing)
- enable mock scrollbars
- disable smooth scrolling
- don't run scrollbar tests on Android

The composited scrollbar button test is disabled due to a bug in how
the mock theme reports its button sizes, which throws off the region
detection in ScrollbarLayerImplBase::IdentifyScrollbarPart (filed
crbug.com/974063 for this issue).

Change-Id: Ie1a762a5f6ecc264d22f0256db68f141fc76b950

Bug: 954007
Change-Id: Ib258e08e083e79da90ba2e4e4216e4879cf00cf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1652741
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669086}