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This adds integration tests for Showcast actions on elements in an
iFrame.

The helper has been rewritten to support selecting elements in iFrames.

Bug: b/143942385
Change-Id: Ida4045364de9bbbcfd27057d9ecb8cb5e8dc8284
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2082564
Commit-Queue: Sandro Maggi <sandromaggi@google.com>
Reviewed-by: Clemens Arbesser <arbesser@google.com>
Cr-Commit-Position: refs/heads/master@{#746785}