Introduce CryptAuthGCMManager, which handles GCM interactions for CryptAuth.

components/proximity_auth/cryptauth/BUILD.gn

Index: components/proximity_auth/cryptauth/BUILD.gn
diff --git a/components/proximity_auth/cryptauth/BUILD.gn b/components/proximity_auth/cryptauth/BUILD.gn
index 12e48c8ed51d730e763f879b131d6c3b932730d8..b2e210fcfc3655796709ab5ffea80d24371bfe7a 100644
--- a/components/proximity_auth/cryptauth/BUILD.gn
+++ b/components/proximity_auth/cryptauth/BUILD.gn
@@ -23,6 +23,10 @@ source_set("cryptauth") {
     "cryptauth_enrollment_manager.h",
     "cryptauth_enrollment_utils.cc",
     "cryptauth_enrollment_utils.h",
+    "cryptauth_gcm_manager.cc",
+    "cryptauth_gcm_manager.h",
+    "cryptauth_gcm_manager_impl.cc",
+    "cryptauth_gcm_manager_impl.h",
     "pref_names.cc",
     "pref_names.h",
     "secure_message_delegate.cc",
@@ -36,6 +40,7 @@ source_set("cryptauth") {
   deps = [
     "//base",
     "//crypto",
+    "//components/gcm_driver",
     "//components/proximity_auth/logging",
     "//google_apis",
     "//net",
@@ -79,6 +84,7 @@ source_set("unit_tests") {
     "cryptauth_device_manager_unittest.cc",
     "cryptauth_enroller_impl_unittest.cc",
     "cryptauth_enrollment_manager_unittest.cc",
+    "cryptauth_gcm_manager_impl_unittest.cc",
     "fake_secure_message_delegate_unittest.cc",
     "sync_scheduler_impl_unittest.cc",
   ]
@@ -88,6 +94,7 @@ source_set("unit_tests") {
     ":test_support",
     "//base:prefs_test_support",
     "//base/test:test_support",
+    "//components/gcm_driver:test_support",
     "//google_apis:test_support",
     "//net:test_support",
     "//testing/gtest",