Oilpan: Use weak pointers in StyleSheetContents caches.

This fixes use-after-free that could occur when StyleSheetContents
dies when an allocation happens during insertion of a new
StyleSheetContents. Using the built-in weak processing this will
not happen because the iterator used during insertion will keep
the StyleSheetContents alive.

R=erik.corry@gmail.com, haraken@chromium.org, vegorov@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/185403016

git-svn-id: svn://svn.chromium.org/blink/trunk@168444 bbb929c8-8fbe-4397-9dbb-9b2b20218538