Revert "Add explicit flag for compositor scrollbar injected gestures"

This reverts commit d9a56afcbdf9850bc39bb3edb56d07d11a1eb2b2.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 669086 as the
culprit for flakes in the build cycles as shown on:
https://analysis.chromium.org/p/chromium/flake-portal/analysis/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyQwsSDEZsYWtlQ3VscHJpdCIxY2hyb21pdW0vZDlhNTZhZmNiZGY5ODUwYmMzOWJiM2VkYjU2ZDA3ZDExYTFlYjJiMgw

Sample Failed Build: https://ci.chromium.org/buildbot/chromium.chromiumos/linux-chromeos-rel/25818

Sample Failed Step: content_browsertests on Ubuntu-16.04

Sample Flaky Test: ScrollLatencyScrollbarBrowserTest.ScrollbarThumbDragLatency

Original change's description:
> Add explicit flag for compositor scrollbar injected gestures
> 
> 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}


Change-Id: Icc743e48fa740fe27f0cb0cfa21b209a696f518c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 954007
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660114
Cr-Commit-Position: refs/heads/master@{#669150}
void InputHandlerProxy::ReconcileElasticOverscrollAndRootScroll() {
  if (scroll_elasticity_controller_)
    scroll_elasticity_controller_->ReconcileStretchAndScroll();
}

void InputHandlerProxy::ReconcileElasticOverscrollAndRootScroll() {
  if (scroll_elasticity_controller_)
    scroll_elasticity_controller_->ReconcileStretchAndScroll();
}
