Move create() methods from StylePropertySet to MutableStylePropertySet.

Move the two create() methods that create MutableStylePropertySets from StylePropertySet
to MutableStylePropertySet, and make them actually return MutableStylePropertySets.
Switch to using {Pass,}RefPtr<MutableStylePropertySet> where appropriate to keep things building.

This is a backport from WebKit r148395. Patch by Andreas Kling and reviewed by Antti Koivisto.

Review URL:

git-svn-id: svn:// bbb929c8-8fbe-4397-9dbb-9b2b20218538
22 files changed