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Add local files as one of the options in the sources dropdown. When it
is selected, a file picker dialog is opened via MediaRouterViewsUI. After
selecting a file, the user can select a sink to start casting.

The dialog has a functionality to close itself on blur, which will be
disabled while a file picker is open, and when the file is opened in a
new tab, to prevent the dialog from closing at those moments.

Bug: 842787
Change-Id: I3e9ebf0b1958c026f418111feb792a9d94fce533
Reviewed-on: https://chromium-review.googlesource.com/c/1299657
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607337}