Now that most of the implementation is complete, this change revisits
our naming and namespacing strategy. An obvious issue is that all our
code had "remote" or "remoting" in its class/variable names, which is
redundant. Also, some code was put in a media::remoting namespace, while
other code was in the media namespace with "remoting" in the class name.
Finally, some code was in subdirectories, but the code base did not turn
out to be so large to warrant that.

This change places all code in the src/media/remoting dir in the
media::remoting namespace, makes some minor style fixes, and tweaks
comments around touched lines of code. The following classes have been
renamed:

1. RemoteRendererImpl --> CourierRenderer because it is an impl of
media::Renderer that maps all operations and callbacks into two-way RPC
messaging, to give the local media pipeline the illusion of having a
local Renderer, but with rendering actually occurring remotely.

2. RemoteDemuxerStreamAdapter --> DemuxerStreamAdapter: Removed
redundant "Remote" from the name since this class is disambiguated by
being in the media::remoting namespace.

3. RemotingSourceImpl --> SharedSession because it is an impl of
media::mojom::RemotingSource that is shared by multiple "clients."

4. RemotingSinkObsever --> SinkAvailabilityObserver, to be more-specific
about its purpose.

5. RemotingRendererFactory --> AdaptiveRendererFactory because this
RendererFactory "wraps" the normal RendererFactory to create either
local or remoting renderers depending on the current situation.

6. RemotingRendererController --> RendererController: Removed redundant
"Remoting" from the name because this class is disambiguated by being
in the media::remoting namespace.

7. Miscellaneous similar renames in "fake" classes and other testing-
only code.

Note: The RemotingCdm* classes were not changed because there is current
WIP to merge-in more implementation. Renaming there will occur in a
later change.

BUG=643964

Review-Url: https://codereview.chromium.org/2643253003
Cr-Commit-Position: refs/heads/master@{#445880}