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On both desktop and Android, the Push API will now be backed by
InstanceID tokens instead of GCM registrations.

This should have no compatibility impact, since GCM was already
silently converting Push API registrations to InstanceID format
server-side.

Part of a series of patches:
1. https://codereview.chromium.org/1832833002 adds InstanceIDWithSubtype
2. https://codereview.chromium.org/1830983002 adds JNI bindings
3. https://codereview.chromium.org/1829023002 adds fake and test
4. https://codereview.chromium.org/1899753002 fixes strict mode violations
5. https://codereview.chromium.org/1854093002 enables InstanceID by default
6. https://codereview.chromium.org/1953273002 extends the GCMKeyStore
7. https://codereview.chromium.org/1923953002 integrates IIDs with crypto
8. https://codereview.chromium.org/2111973002 adds IID subtypes on desktop
9. this patch

Also depends on
- https://codereview.chromium.org/1945753002 transactional SW userdata
- https://codereview.chromium.org/1944863002 fix sender ID storage

BUG=589461, 533498

Review-Url: https://codereview.chromium.org/1851423003
Cr-Commit-Position: refs/heads/master@{#417323}