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This CL adds test API that will be used by integration and e2e browser
tests to manipulate the Views Cast dialog.

Bug: 900243
Change-Id: I0dc0c4ce08a3f9b78764459b6298502d4a8e0901
Reviewed-on: https://chromium-review.googlesource.com/c/1284211
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607983}