Oilpan: cleanup based on review comments after removal of TreeShared.

There are a couple of comments (in particular SVG stuff) that I
have not addressed. I have filed bug reports for those to make
sure that I get around to revisiting them.

R=haraken@chromium.org, kouhei@chromium.org, tkent@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/267303004

git-svn-id: svn://svn.chromium.org/blink/trunk@173645 bbb929c8-8fbe-4397-9dbb-9b2b20218538