aw: Fix capturePicture to ignore scroll offset
aw: Fix capturePicture to ignore scroll offset

capturePicture is supposed to ignore any scroll offsets. The bug is that
need to call DidChangeRootLayerScrollOffset when scroll offset changes.

No point in being fancy or optimizing a deprecated api. Just straight up
call DidChangeRootLayerScrollOffset twice.

BUG=520276

Review URL: https://codereview.chromium.org/1284413003

Cr-Commit-Position: refs/heads/master@{#343116}
(cherry picked from commit 28bcb65ebc5f5c67577a813437265d7e6b885620)
TBR=boliu@chromium.org

Review URL: https://codereview.chromium.org/1288843002 .

Cr-Commit-Position: 62bea078bfe806fd4d8dc2d3869914b6c8093498@{#304}
Cr-Branched-From: ef8f355264e75087b06072b03d328c50bf72843a@{#338390}

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