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The top bar was still taking up space even though it is not needed in
fullscreen. This also fixes a bug where pressing the back button did not
exit fullscreen in the shell.

Bug: 1029894
Change-Id: I215e8e1066670ba2e57d83114537b95c70186d99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1946629Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720761}