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Currently, we only set WindowEventTarget to DirectManipulationHelper
when receive DM_POINTERHITTEST. But the crash shows OnContentUpdate
may come before DM_POINTERHITTEST. To prevent the crash we set the
WindowEventTarget when DirectManipulationHelper create.

Bug: 839338
Change-Id: I2921277178771f9124a37325a8dff47196500322
Reviewed-on: https://chromium-review.googlesource.com/1042381Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555904}