Enable Oilpan by default for websocket/

This CL basically just drops WillBe types from websocket objects. Non-trivial parts are:

- Changed MockWebSocketChannelClient from RefCountedGarbageCollected to GarbageCollectedFinalized since it doesn't need to be RefCounted.

- Marked AllowCrossThreadAccessWrapper as STACK_ALLOCATED, and intentionally allowed to use a raw pointer to T*. Ideally this should be Member<T> if T inherits from GarbageCollected, but I don't want to introduce another template magic just to distinguish Member<T> from T* on stack-allocated objects. Added a comment about it.

BUG=340522

Review URL: https://codereview.chromium.org/429293003

git-svn-id: svn://svn.chromium.org/blink/trunk@179336 bbb929c8-8fbe-4397-9dbb-9b2b20218538