FontPlatformDataCache uses OwnPtr instead of raw pointer.

HashMap can handle OwnPtr, so we don't have to delete an element in HashMap
manually.

Review URL: https://codereview.chromium.org/26864002

git-svn-id: svn://svn.chromium.org/blink/trunk@159453 bbb929c8-8fbe-4397-9dbb-9b2b20218538