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This is a reland of 50fbc87a

The CL got reverted due to flakiness on Win7. From the logs it appears
that the flakes were caused by test content sometimes extending beyond
window bounds, resulting in incorrect hit test results.

To address this issue, existing tests have been rewritten to use smaller
content. I ran the rewritten tests 100x on a Win7 VM and saw 0 failures.

Several cleanups were made along the way:

- Update comments and code to reflect the fact that
BrowserAccessibilityManager.HitTest takes a point in frame coordinates.

- Increase device scale factor in scaled tests from 1.25 back to 2.

- AccessibilityHitTestingBrowserTest.HitTestingInIframes: This test was
almost identical to AccessibilityHitTestingZoomBrowserTest.HitTest, with
the addition of testing with a different event. Combined the two into a
single test: AccessibilityHitTestingBrowserTest.HitTest.

- AccessibilityHitTestingCrossProcessBrowserTest.*: these two tests were
also nearly identical. Combined into a single test which validates both
the frame tree and the scrolling case with the same content.

- AccessibilityHitTestingBrowserTest.*WithPinchZoom: these tests were
not actually exercising accessibility hit testing paths - they were
generating input events which got sent directly to Blink. Replaced these
with tests that exercise accessibility hit test paths in the presence of
pinch zoom.

Original change's description:
> Parameterize existing accessibility hit-testing tests
>
> https://crrev.com/c/2117330 added tests for cross-platform
> accessibility hit testing code using parameterized combinations of
> simulated device scale factor and use-zoom-for-dsf on or off. This CL
> expands the parameterization to other existing hit-testing tests.
>
> To fix issues running these tests on cast_shell_linux I also made two
> changes to the tests themselves:
> - Reduce the scale factor from 2 to 1.25
> - Relocate one test point to be within the screen size
>
> Bug: 1007488
> Change-Id: I715270064365458ac9ec69f3c3ac81a674d2718c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135017
> Commit-Queue: Kevin Babbitt <kbabbitt@microsoft.com>
> Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#756884}

Bug: 1007488, 1069850
Change-Id: I01fcb7d2b7f6a764ccd76da3e97778dfc184960e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2140198
Commit-Queue: Kevin Babbitt <kbabbitt@microsoft.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758545}