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.

R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/15864004

git-svn-id: svn://svn.chromium.org/blink/trunk@151009 bbb929c8-8fbe-4397-9dbb-9b2b20218538
22 files changed