webview: Remove unnecessary call to dirty the draw properties.
If the new delegated scroll offset moves any of the layers, they will
call NoteLayerPropertyChangedForSubtree() which sets the dirty bit for
the active tree's draw properties. So there's no need to dirty the
draw properties again in OnRootLayerDelegatedScrollOffsetChanged().
R=boliu
BUG=531746
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
Review URL: https://codereview.chromium.org/
1351673002
Cr-Commit-Position: refs/heads/master@{#349217}