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It has been essentially unused for a while.
Apart from deleting the code, this CL contains the following changes:
- In ChromeGcmListenerService: stop passing incoming
  GCM messages to the cacheinvalidation code.
- In ProcessInitializationHandler: stop initializing cacheinvalidation's
  Logger class.
- Create a dummy GcmRegistrationTaskService: This service (part of the
  cacheinvalidation library) is listed in the AndroidManifest.xml as
  exported, so it's part of Chrome's public API and can't just be
  removed. Adding an empty dummy implementation lets us keep the
  manifest entry without the actual implementation from the library.

Bug: 1099672
Change-Id: I7e17415cd33cd76b864bd9e6f310f6c52bdf5503
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274884
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787662}