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This way of writing it was considered in
https://codereview.chromium.org/19106007#msg30 but the direct
scroll_delta_ was felt to be slightly cleaner and with no behavior
difference unless "you had some weird stateful scroll offset delegate."
Pinch viewport mode indeed introduced state inside
LayerScrollOffsetDelegateProxy to buffer the values provided by the
inner and outer viewports before calling the delegate, so this is now
more correct.  Specifically, it's now necessary to echo back to the
delegate the values forced by its getter, or they may be clobbered with
older values later.

NOTRY=true
BUG=426891

Review URL: https://codereview.chromium.org/665233004

Cr-Commit-Position: refs/heads/master@{#302711}