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Checking that the fullscreen element is non-null implicitly means that
fullscreen has already been granted.

Verified via:
out/Release/chrome --register-pepper-plugins="out/Release/libppapi_example_mouse_lock.so;application/x-ppapi-example-mouse-lock"

BUG=1097816

Change-Id: I125da5db68346f5cc90279110ce676c70cf27c0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488124
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819148}