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This item is only shown when in fullscreen, and allows users to exit
fullscreen using the mouse (for users who don't have a keyboard
attached).

This is still not foolproof (as it won't work on a site that captures
the right-click menu) but should work in the majority of cases.

BUG=594868
TEST=https://permission.site. Click Fullscreen. Right-click background; should
     see "Exit full screen ... Esc".
TEST=F11 to go fullscreen. Right-click background; should see
     "Exit full screen ... F11".

Review-Url: https://codereview.chromium.org/2116973002
Cr-Commit-Position: refs/heads/master@{#404589}