This is a reland of d44248f2a896421c1382c1dbbf0d7e1a9b798470

Original change is reverted by findit due to test flaky.
The flaky test is:
fast/events/pointerevents/mouse-pointer-capture-in-iframe.html

This is reland with fixing tests.

Original change's description:
> Make pointer capture work in same origin frame
>
> We used to send mouse event to the subframe before apply the pointer
> capture target, and it causes the pointer capture doesn't work when
> capture to a outer frame target and move to inner frame.
>
> This CL changes three things:
> 1. 'Rename' the |capturing_mouse_events_element_| to
> |capturing_subframe_element_| as it only used for the frame capture
> (There is plan to remove the frame capture logic once we have pointer
> capture work correctly)
> 2. On HandleMouse*Event, instead of always perform a hit test, we use
> either frame capture target or pointer capture target to re-construct
> the HitTestResult.
> 3. When using the capture target, update the hover active state for
> capture target.
>
> This change makes captured pointer event sent correctly when over
> same origin frame, and also decrease the hit_test_count because of
> frame capturing.
>
> Note that after this change, we still NOT allow set/release pointer
> capture across same-origin frame as the pointer id and active statue
> is per frame.
>
> See design doc:
> https://docs.google.com/document/d/1cOZu98UuKk5bdARUQKmj2Q8YoEMpd9l78T0k-cf5ttc/
>
> This change is under a blink flag UnifiedPointerCaptureInBlink.
>
> Change-Id: I61c6a02086535d2a145df9d414df0bdc9673101e
> Bug: 936190, 919908
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1446556
> Commit-Queue: Ella Ge <eirage@chromium.org>
> Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
> Reviewed-by: David Bokan <bokan@chromium.org>
> Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#640055}

Bug: 936190, 919908
Change-Id: Icc374c16b1aee0b9125593801c8787f28c7db554
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1520941
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641316}