Explicitly give entered iframes focus

Bug: b:112665662
Change-Id: I6e0d7661d08ba6560b3a6591db1a29073462b116
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577919
Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653034}