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This patch is for set the scroll_event_phase_ kNone as default
in ctor. It hits DCHECK in MakeWebMouseWheelEventFromUiEvent()
if the field not set.

Bug: 647140

Change-Id: I3fd523bd84788957d0d05770fa70503be619e4ad
Reviewed-on: https://chromium-review.googlesource.com/998755Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548681}