Revert of Oilpan: Prepare to make ExecutionContext GarbageCollectedMixin. (https://codereview.chromium.org/318023002/)

Reason for revert:
plugins/document-open.html is crashing on debug layout bots

Original issue's description:
> Oilpan: Prepare to make ExecutionContext GarbageCollectedMixin.
> 
> We creates closures with ExecutionContexts in LocalFileSystem. The
> ExecutionContext needs to be a CrossThreadPersistent data member of
> closure. This CL adds ParamStorageTraits specialized to garbage-collected
> classes.
> 
> This is a preparation to remove RefCounted from Node.
> 
> BUG=357163
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175734

TBR=oilpan-reviews@chromium.org,haraken@chromium.org,tkent@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=357163

Review URL: https://codereview.chromium.org/319333002

git-svn-id: svn://svn.chromium.org/blink/trunk@175741 bbb929c8-8fbe-4397-9dbb-9b2b20218538