Fix Chrome linker errors in GN build.

This fixes all non-RTC-related linker errors for Chrome.

[email protected]

Review URL: https://codereview.chromium.org/533743004

Cr-Commit-Position: refs/heads/master@{#292978}
diff --git a/components/gcm_driver/BUILD.gn b/components/gcm_driver/BUILD.gn
index 23421dc..6ee0dc6 100644
--- a/components/gcm_driver/BUILD.gn
+++ b/components/gcm_driver/BUILD.gn
@@ -18,6 +18,8 @@
     "gcm_client_factory.h",
     "gcm_client_impl.cc",
     "gcm_client_impl.h",
+    "gcm_connection_observer.cc",
+    "gcm_connection_observer.h",
     "gcm_driver.cc",
     "gcm_driver.h",
     "gcm_driver_android.cc",