[oilpan]: Make CSSValue a true garbage collected class in oilpan.

I have added a persistent keepAlive in the CSSImageGenerator to ensure that it stays alive as long as it has RenderObject clients.

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=341815

Review URL: https://codereview.chromium.org/248893003

git-svn-id: svn://svn.chromium.org/blink/trunk@172494 bbb929c8-8fbe-4397-9dbb-9b2b20218538