Fix pointer lock / mouse lock on linux failing to send mouse moves.

Original implementation was flawed when unlocking the mouse pointer.
While being warped back to the unlocked location it was possible that
move events already in flight would be received and update
unlocked_global_mouse_position_. That would cause the eventual move
event from the warp to not be detected as having been processed, and
suppress all movementX/Y data indefinitely.

BUG=324203

TEST=Manual test please: On Linux visit pointer lock sites such as http://media.tojicode.com/q3bsp/, http://scheib.github.io/HTMLMisc/PointerLockAndFullscreen.html, and enter and exit pointer lock several times verifying that the mouse movement continues to have an effect.
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/105993002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238991 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238991