Move CSSOM classes to using MutableStylePropertySet over StylePropertySet.

Moved ensureCSSStyleDeclaration() and ensureInlineCSSStyleDeclaration() from StylePropertySet
to MutableStylePropertySet.

StyleRule & co now return a MutableStylePropertySet* from mutableProperties() to aid with...

CSSStyleDeclaration now only deals with MutableStylePropertySets. This was already the case
in practice, we just didn't have any compile-time foot-shooting protection.
In other words, if there's a CSSOM wrapper for a given StylePropertySet, it's guaranteed to
be mutable, and the pointer types now reflect this.

This is a backport from WebKit r148396. Patch by Andreas Kling reviewed by Anders Carlsson.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/15923002

git-svn-id: svn://svn.chromium.org/blink/trunk@151073 bbb929c8-8fbe-4397-9dbb-9b2b20218538
14 files changed