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Reason for revert:
The change broke several Chromium OS interactive_ui_tests:

ExtensionPointerLockAccessFail
ExtensionPointerLockAccessPass
EscapingMouseLock
MouseLockSilentAfterTargetUnlock
TestTabDoesntExitMouseLockOnSubFrameNavigation
TestTabExitsMouseLockOnGoBack
TestTabExitsMouseLockOnNavigation
PointerLock_PointerLockLostWithFocus

that were using the prefixed API (""Uncaught TypeError: undefined is not a function").

See
http://build.chromium.org/p/chromium.webkit/builders/Linux%20ChromiumOS%20Tests%20%282%29/builds/735/steps/interactive_ui_tests/logs/stdio

Original issue's description:
> Remove prefixed Pointer Lock API.
> 
> The Pointer Lock API http://www.w3.org/TR/pointerlock has already been
> implemented without prefix, this change removes the 'webkit' prefixed
> API endpoints.
> 
> Intent to Deprecate and Remove:
> https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/tN8fWRtG0MI
> 
> BUG=398457
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179172

TBR=abarth@chromium.org,scheib@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=398457

Review URL: https://codereview.chromium.org/424213007

git-svn-id: svn://svn.chromium.org/blink/trunk@179186 bbb929c8-8fbe-4397-9dbb-9b2b20218538