Optimize comparison of DataRefs

This follows the following comment in RenderStyle::visualInvalidationDiff():
    // Note, we use .get() on each DataRef below because DataRef::operator== will do a deep
    // compare, which is duplicate work when we're going to compare each property inside
    // this function anyway.

- Changed some unnecessary deep comparisons of DataRefs before
  comparing the fields of the DataRefs in SVGRenderStyle::diff()
  to .get() comparisons.

- Added .get() comparison before multiple field comparisons.

Review URL: https://codereview.chromium.org/260913002

git-svn-id: svn://svn.chromium.org/blink/trunk@173100 bbb929c8-8fbe-4397-9dbb-9b2b20218538