Introduce DEFINE_CUSTOM_FONT_DATA_TYPE_CASTS, and use it

As a step to use toFoo, this cl introduces DEFINE_CUSTOM_FONT_DATA_TYPE_CASTS, and use it.
Additionally this cl clean up static_cast<TextCloneCSSValue*> with toTextCloneCSSValue().

BUG=309516

Review URL: https://codereview.chromium.org/453203003

git-svn-id: svn://svn.chromium.org/blink/trunk@179898 bbb929c8-8fbe-4397-9dbb-9b2b20218538