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So that we can rely on IPC_MESSAGE_CLASS(msg) check to
determine which messages we want to relay onto another thread.

Mostly just shuffling,
- Separated EmbeddedWorker messages out of service_worker_messages.h into embedded_worker_messages.h
- Added a new message class for that
- Created a new directory content/common/service_worker (and OWNERS file) as now we have multiple header files for SW under common/

BUG=313530

Review URL: https://codereview.chromium.org/127583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243871 0039d316-1c4b-4281-b951-d872f2087c98