Only unregister touch action handler conditionally

When a RenderObject with a non-default touch action is about to be
destroyed, we remove the fake event handler that implements the touch
action from the event handler registry. However the handler may have
already been removed previously by the host Document (e.g., from
Document::open()), so this code can cause the same handler to get
removed twice.

This patch fixes the issue by only removing the handler conditionally
if it is still there.

BUG=419420

Review URL: https://codereview.chromium.org/647503002

git-svn-id: svn://svn.chromium.org/blink/trunk@183743 bbb929c8-8fbe-4397-9dbb-9b2b20218538