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This avoids accidental layering violations, by enforcing use of
GCMDriver::GetInstanceIDHandlerInternal (which is commented as "These
methods must only be used by the InstanceID system") to get access to
InstanceIDHandlers.

BUG=none

Review-Url: https://codereview.chromium.org/2610933002
Cr-Commit-Position: refs/heads/master@{#441425}