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This change introduces a new service (SharedWorkerFactory) exported
from renderer processes to support the instantiation of shared worker
instances.

The SharedWorkerFactory is instantiated from within the
SharedWorkerServiceImpl::TryReserve function, which runs on the UI
thread. This way we defer connecting to that service until we want
to create a shared worker. It also allows the unit tests to swap in
a mock implementation of SharedWorkerFactory.

In a future CL, it'll be possible to remove the
SharedWorkerMessageFilter. Even though that class now no longer
filters any Chrome IPC messages, its other functionality is still
needed. Similarly, we still need to allocate a routing ID for the
shared worker even though that is not needed by the shared worker
infrastructure. Dev Tools uses the routing ID to help lookup the
shared worker, so that'll need to change before we clean this up.

Also, in a future CL, it'll be possible to move the shared worker
service code to run entirely on the UI thread. This will allow a lot
of code simplification as we'll no longer have to deal w/ the race
conditions during process reservation. A bunch of complexity will
disappear. To try to keep this CL manageable, I chose to postpone
the threading changes to a follow-up CL.

The lifetime of the SharedWorker instance in the renderer process is
a bit squirrely thanks to how WebSharedWorker manages its lifetime.
This CL does not attempt any changes there.

Bug: 612308
Change-Id: Ifc355d42d0c869519acee853c953342a2bcf691a
Reviewed-on: https://chromium-review.googlesource.com/659139Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kinuko Yasuda (slow) <kinuko@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Darin Fisher <darin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502910}