Revert 182406 "WebThreadSupportingGC's destructor should have a ..."

This broke WebDatabase tests in oilpan builds:
https://code.google.com/p/chromium/issues/detail?id=416772

> WebThreadSupportingGC's destructor should have a safe point scope
> 
> WebThreadSupportingGC's destructor can block because WebThread's destructor blocks until all the tasks are processed. Thus we should add a SafePointScope to WebThreadSupportingGC's destructor. This CL does it.
> 
> The reason the current WebThreadSupportingGC is working without having the SafePointScope is that the caller side of the WebThreadSupportingGC uses a TaskSynchronizer to avoid the dead lock that the WebThreadSupportingGC may cause.
> 
> After landing this CL, we can remove the TaskSynchronizers from the caller side.
> 
> BUG=340522
> 
> Review URL: https://codereview.chromium.org/590943003

TBR=haraken@chromium.org

Review URL: https://codereview.chromium.org/592393002

git-svn-id: svn://svn.chromium.org/blink/trunk@182502 bbb929c8-8fbe-4397-9dbb-9b2b20218538