V8DOMActivityLogger should be owned by DOMWrapperWorld

- Currently V8DOMActivityLogger is owned by a static hash map and cached on V8PerContextData. This is mis-designed. Given that an ActivityLogger should conceptually be associated with a world, V8DOMActivityLogger should be owned by DOMWrapperWorld. This CL makes the change.

- The reason why we were caching V8DOMActivityLogger on V8PerContextData is that DOMWrapperWorld::current() was slower than V8PerContextData::from(). Recently I optimized DOMWrapperWorld::current() and now it's as fast as V8PerContextData::from(). So I think it's now OK to make this change.

Review URL: https://codereview.chromium.org/264853002

git-svn-id: svn://svn.chromium.org/blink/trunk@173136 bbb929c8-8fbe-4397-9dbb-9b2b20218538