CSSValue patch 1 with const CSSValue& and move operators

CSSValue Immediates: Add move operators to CSSValue and NullableCSSValue

Add move operators (both constructors and operator= methods) to CSSValue
and NullableCSSValue in an attempt to reduce ref-churn. Although this is
against the blink style guide, it was done as an experiment to see if
performance-related issues were being caused from excessive copy
operations.

Depends on crrev.com/1238943004.

This is part of a larger refactor of CSSValue to introduce immediates,
discussed on the bug.

BUG=486252
COMMIT=false