Extract GCMClient data types into separate gcm_types.h

Extract GCMClient data types into separate gcm_types.h

This patch moves Result, MessageData, OutgoingMessage, IncomingMessage
and SendErrorDetails out of GCMClient and into a new gcm_types.h file
that can be included in cases where GCMClient is not being used, but it
it still desirable to communicate using the same data types, for
example the upcoming Android GCM implementation will share these.

BUG=350384

[johnme, 2014-05-08 15:14:06]

Hey Jian, is this the kind of thing you meant on
https://codereview.chromium.org/278493002#msg4 by "those types defined in
GCMClient, like Result, IncomingMessage and OutgoingMessage should be considered
to move to other place. Maybe we could move these to google_apis/gcm/public"?

[johnme, 2014-11-25 14:31:16]

Abandoned - no longer necessary now that GCMClient.h moved into the
components/gcm_driver