SVG: Mark wrong ENABLE(OILPAN) ifdefs

and remove an unnecessary null-check when iterating through weak collection.

BUG=357163

Review URL: https://codereview.chromium.org/305463002

git-svn-id: svn://svn.chromium.org/blink/trunk@174803 bbb929c8-8fbe-4397-9dbb-9b2b20218538