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This patch fixes MouseEvent.layerX and layerY when root-layer-scrolls is
enabled. The layer coordinates are calculated by subtracting from the
document location the layer offsets of each parent layer starting from
the current layer's parent. This yields the offset within the layer.

The repeated subtraction gets us back to the absolute coordinate space.
Pre-RLS, this was the same as document/content coordinates. With RLS
turned on, the LayoutView is the same size as the frame and clips and
scrolls its content. Thus, transforming through it yields frame
coordinates. Therefore, the change is to start from the event's absolute
location. With RLS off, the conversion yields a no-op so there's no
change. When RLS is on this accounts for the LayoutView's scroll offset.

Bug: 781431
Change-Id: Ib25859effbfb71516e5edb5173154c807ea98314
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_root_layer_scrolls
Reviewed-on: https://chromium-review.googlesource.com/798832
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521279}