CSS Animations: Compare values for -webkit-clip-path correctly

Values of this property were being compared using pointer equality, which
meant that transitions could be started despite values being otherwise
identical.

BUG=322178

Review URL: https://codereview.chromium.org/103453008

git-svn-id: svn://svn.chromium.org/blink/trunk@164053 bbb929c8-8fbe-4397-9dbb-9b2b20218538