ResourceLoaderEditToolbarModule::cssSerializeString() → CSSMin::serializeStringValue()

ResourceLoaderEditToolbarModule::cssSerializeString() → CSSMin::serializeStringValue()

ResourceLoaderEditToolbarModule is clearly the wrong place for something so
generic, so this method needs a new home. We can either introduce a new class
or find a suitable existing home. I think CSSMin is suitable. It has public
methods that do things that are closely related, like `encodeStringAsDataURI`
and `buildUrlValue`.

Change-Id: Icc6dfb8f47199e6188dd71948f4645baee085e51