Oilpan: Move StyleSheetList to the oilpan heap using transition types.

This is in preparation for moving StyleSheet. This object has vectors
of style sheets that I want to turn into heap vectors and trace. Doing
the containers first to make the style sheet patch smaller.

R=haraken@chromium.org, wibling@chromium.org
BUG=341815

Review URL: https://codereview.chromium.org/168533003

git-svn-id: svn://svn.chromium.org/blink/trunk@167518 bbb929c8-8fbe-4397-9dbb-9b2b20218538