Flakiness dashboard link: https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=virtual%2Fthreaded%2Ffast%2Fevents%2Fpinch

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3af420d0f4d2c370a477186db5b90e0fe7db9b11

commit 3af420d0f4d2c370a477186db5b90e0fe7db9b11
Author: David Bokan <bokan@chromium.org>
Date: Fri Aug 24 16:41:35 2018

Fix pinch zoom tests on Mac

These tests:
virtual/threaded/fast/events/pinch/pinch-zoom-into-center.html
virtual/threaded/fast/events/pinch/pinch-zoom-pan-position-fixed.html
pinch-zoom-pan-within-zoomed-viewport.html

were breaking because Mac doesn't support touch events and the pinch
was being done using touch. I've changed to use a touchpad source as
this doesn't affect the meaning of the tests as well as some minor
cleanups.

While here, I also rewrote a number of tests to use gesture-util.js and
the async/await style which is more readable and easier to produce
non-flaky tests.

TBR=nasko@chromium.org

Bug:  854199 , 873435 , 877494 , 877361 
Change-Id: I19f6e199fea694013d3dceaeb32c881059301f1d
Reviewed-on: https://chromium-review.googlesource.com/1185238
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Sahel Sharify <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585871}
[modify] https://crrev.com/3af420d0f4d2c370a477186db5b90e0fe7db9b11/third_party/WebKit/LayoutTests/TestExpectations
[modify] https://crrev.com/3af420d0f4d2c370a477186db5b90e0fe7db9b11/third_party/WebKit/LayoutTests/fast/scroll-behavior/overscroll-behavior.html
[delete] https://crrev.com/2150cdb8fe59c54e803aec058c661b9e0b953e4d/third_party/WebKit/LayoutTests/virtual/threaded/animations/invisible-composited-animations-prevent-scroll-expected.html
[modify] https://crrev.com/3af420d0f4d2c370a477186db5b90e0fe7db9b11/third_party/WebKit/LayoutTests/virtual/threaded/animations/invisible-composited-animations-prevent-scroll.html
[modify] https://crrev.com/3af420d0f4d2c370a477186db5b90e0fe7db9b11/third_party/WebKit/LayoutTests/virtual/threaded/fast/events/pinch/gesture-pinch-zoom-scroll-bubble.html
[modify] https://crrev.com/3af420d0f4d2c370a477186db5b90e0fe7db9b11/third_party/WebKit/LayoutTests/virtual/threaded/fast/events/pinch/pinch-zoom-pan-position-fixed.html
[modify] https://crrev.com/3af420d0f4d2c370a477186db5b90e0fe7db9b11/third_party/WebKit/LayoutTests/virtual/threaded/fast/events/pinch/pinch-zoom-pan-within-zoomed-viewport.html