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The GCM handler doesn't need to create the GCM token. Splitting this interface
simplifies the code. Additionally, we can now always create the gcm handler,
even in reduced mode.

GetGCMToken() is now a global function, and still called only after browser start.

This is safer, because the handler is accessed by
another keyed service. Before this CL, there was no guarantee that the gcm handler
would be created before it's accessed by gcm_profile_service_factory.cc

Change-Id: I27ab837f4294f450ee11dfaa9a833ba90cf3c83a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1690712
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706646}