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This patch addes a window property to make the Assistant answer card
scrollable in the embedded UI.
Changes:
1. Refactor current code so that app_list_view can access the window
   property.
2. Modify app_list_view event targeter to handle Assistant answer card
   events.

Bug: 924624
Test: manual
Change-Id: Iad9650ca98589793ff065574b3971aa97c3ec3f5
Reviewed-on: https://chromium-review.googlesource.com/c/1482591Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634774}