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This does not add any new functionality, but sets the stage for things
like requesting the host to show a file picker.

Bug: 679313
Change-Id: Id0a25d038f274584dfd6661df50658437ad29258
Reviewed-on: https://chromium-review.googlesource.com/c/1265028
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605033}