Mojo: Rename PlatformChannelHandle to PlatformHandle, etc.

Mojo: Rename PlatformChannelHandle to PlatformHandle, etc.

We're going to need handles to things other than "channels", so rename
PlatformChannelHandle to PlatformHandle.

Rename/refactor PlatformChannel to ScopedPlatformHandle (because that's
what it really was), and make it move-only.

Adapt everything else to fit this model.

R=darin@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=244742

[viettrungluu, 2014-01-13 23:56:09]

More notes:
- I'll need to expose PlatformHandle/ScopedPlatformHandle to the embedder API. I
might move them to the embedder namespace, but I haven't decided yet.
- At that time, possibly embedder.* and (scoped_)platform_handle.* will also
move to a separate directory.
- Probably PlatformChannelPair should have a new name too, but I haven't decided
on one yet.

[darin (slow to review), 2014-01-14 05:57:27]

LGTM

[viettrungluu, 2014-01-14 16:30:48]

Committed patchset #3 manually as r244742 (presubmit successful).