[Offline Pages] Add a GCMAppHandler for offline page prefetch.

components/offline_pages/core/prefetch/BUILD.gn

Index: components/offline_pages/core/prefetch/BUILD.gn
diff --git a/components/offline_pages/core/prefetch/BUILD.gn b/components/offline_pages/core/prefetch/BUILD.gn
index 915f0478d1194794c1f4c2e759f51d7d5390e0f0..79897631a4cc28e3ce255d519dc3ef52c82451c0 100644
--- a/components/offline_pages/core/prefetch/BUILD.gn
+++ b/components/offline_pages/core/prefetch/BUILD.gn
@@ -11,6 +11,8 @@ static_library("prefetch") {
   sources = [
     "generate_page_bundle_request.cc",
     "generate_page_bundle_request.h",
+    "prefetch_gcm_app_handler.cc",
+    "prefetch_gcm_app_handler.h",
     "prefetch_item.cc",
     "prefetch_item.h",
     "prefetch_request_fetcher.cc",
@@ -22,6 +24,8 @@ static_library("prefetch") {
     "prefetch_types.h",
     "prefetch_utils.cc",
     "prefetch_utils.h",
+    "suggested_articles_observer.cc",
+    "suggested_articles_observer.h",
   ]
 
   public_deps = [
@@ -29,8 +33,12 @@ static_library("prefetch") {
   ]
   deps = [
     "//base",
+    "//components/gcm_driver",
+    "//components/gcm_driver/common",
     "//components/keyed_service/core",
+    "//components/ntp_snippets",
     "//components/offline_pages/core",
+    "//components/offline_pages/core:switches",
     "//net:net",
     "//url",
   ]
@@ -55,11 +63,13 @@ source_set("unit_tests") {
     "prefetch_request_test_base.cc",
     "prefetch_request_test_base.h",
     "prefetch_service_impl_unittest.cc",
+    "suggested_articles_observer_unittest.cc",
   ]
 
   deps = [
     ":prefetch",
     "//components/offline_pages/core",
+    "//components/offline_pages/core:test_support",
     "//net:test_support",
     "//testing/gmock",
     "//testing/gtest",