Fix a mouse lock problem on Windows where in clicks outside the web page would also cause mouse loc…

Fix a mouse lock problem on Windows where in clicks outside the web page would also cause mouse lock to happen.

This fixes an issue with the mouse locking code on Windows where in if a page entered a mouse lock state and exited
it via the Esc key, subsequently clicking outside the page would cause the lock prompt to display again. This was because
RenderWidgetHostViewAura grabs capture on all platforms including Windows. On Windows the capture happens elsewhere in the
mouse handler. However when the mouse release comes in it is possible that we are already in the mouse locked state and
hence the capture is never released.

Fix is to release capture on all platforms.

BUG=526378
R=scheib