Revert of Oilpan: Move StyleSheets to the oilpan heap using RefCountedGarbageCollected. (https://codereview.chromium.org/173783002/)

Reason for revert:
fast/dom/shadow/move-style-scoped-to-another-shadowroot-crash.html is crashing

Original issue's description:
> Oilpan: Move StyleSheets to the oilpan heap using RefCountedGarbageCollected.
> 
> Resolve CSSStyleSheet/StyleSheetContents finalization order issue
> by making the client pointers in StyleSheetContents weak. That
> involves going to a hash set instead of a vector and therefore
> we need a hash function for RawPtr.
> 
> The style engine change is in preparation for tracing the contained
> collections of CSSStyleSheet pointers.
> 
> R=haraken@chromium.org, jochen@chromium.org, vegorov@chromium.org
> BUG=
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168030

TBR=haraken@chromium.org,jochen@chromium.org,vegorov@chromium.org,erik.corry@gmail.com,mikhail.pozdnyakov@intel.com,ager@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/178403003

git-svn-id: svn://svn.chromium.org/blink/trunk@168038 bbb929c8-8fbe-4397-9dbb-9b2b20218538