Clean up RenderStyle's NonInheritedFlags

Lets drop the getters and rename the fields to use the modern style. I
didn't rename the noninherited_flags member on RenderStyle itself just
yet so this is a bit easier to review. I'll search and replace the
member in another patch.

Review URL: https://codereview.chromium.org/375783002

git-svn-id: svn://svn.chromium.org/blink/trunk@177640 bbb929c8-8fbe-4397-9dbb-9b2b20218538