Remove duplicated headers from core/ directory where the implementation file includes the same dependencies as the corresponding primary header.

BUG=317231

Review URL: https://codereview.chromium.org/64643009

git-svn-id: svn://svn.chromium.org/blink/trunk@162772 bbb929c8-8fbe-4397-9dbb-9b2b20218538
WorkerMessagingProxy is heavily overloaded for worker<->main thread messaging, and is used on both threads. This is making the code unreadable.

This patch:
- Separates WorkerObjectProxy interface out of WorkerMessagingProxy
- Uses closure when possible
- Adds implementation to WorkerObjectProxy, since it only has a single implementation now

After this patch:
- WorkerObjectProxy is only called on Worker Thread (and relays methods to main thread)
- Most of WorkerMessagingProxy methods are called on Main Thread

Cleanup only, no behavioral changes 

BUG=none

Review URL: https://codereview.chromium.org/80383003

git-svn-id: svn://svn.chromium.org/blink/trunk@162773 bbb929c8-8fbe-4397-9dbb-9b2b20218538