Implement MouseEvent.getModifierState

Move the getModifierState implementation from the KeyboardEvent into
the UIEventWithKeyState object. Expose the getModiferState
method in the MouseEvent.idl.

BUG=532963

Review URL: https://codereview.chromium.org/1350933003

git-svn-id: svn://svn.chromium.org/blink/trunk@202581 bbb929c8-8fbe-4397-9dbb-9b2b20218538
9 files changed