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When mobile viewport is enabled, we have two viewports: the visual
(inner) viewport and the layout (outer) viewport. The clip of the
visible viewport is not in the paint property tree, so we need to
apply the clip separately. The clip was mistakenly removed in
crrev.com/824480.

Also fix a bug before crrev.com/824480: the current scroll offset
of the visual viewport was not considered when applying the visual
viewport clip, which caused invisible shifts reported and visible
shifts not reported.

Bug: 1141739
Change-Id: I425718b1fdfb2689a7f2948c561101d0b476eb7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593180
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837853}