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In this patch, we covert the scale factor from Direct Manipulation
to GesturePinch event. The ctrl+wheel will create by WebViewImpl::
HandleSyntheticWheelFromTouchpadPinchEvent.

Bug: 647140
Change-Id: Ib041d7805daac7e71b2f43872c818a9f36b14abd
Reviewed-on: https://chromium-review.googlesource.com/786348Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542719}