Move UserGestureIndicator to platform and delete UserTypingGestureIndicator

This removes two classes from core/dom/ that didn't appear to belong there.

Though UserTypingGestureIndicator was referenced in Blink, its public methods
were never called, so I deleted it rather than moving it to platform.

R=abarth

Review URL: https://codereview.chromium.org/26490006

git-svn-id: svn://svn.chromium.org/blink/trunk@159178 bbb929c8-8fbe-4397-9dbb-9b2b20218538