Revert of Oilpan: Use weak pointers in StyleSheetContents caches. (https://codereview.chromium.org/185403016/)

Reason for revert:
This patch seems to have made a number of layout tests fail on the oilpan bots. Reverting while I investigate.

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168444

TBR=erik.corry@gmail.com,haraken@chromium.org,vegorov@chromium.org,zerny@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/187473003

git-svn-id: svn://svn.chromium.org/blink/trunk@168458 bbb929c8-8fbe-4397-9dbb-9b2b20218538