Generate and use the real key to sign UMA/UKM uploads.

BUG=906202

Change-Id: I10d2de8a1f30c0a56cc95f69aa6c252f171dca73
Reviewed-on: https://chromium-review.googlesource.com/c/1461089
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630939}

google_apis/google_api_keys.h

@@ -79,6 +79,9 @@ std::string GetNonStableAPIKey();
 void SetAPIKey(const std::string& api_key);
 #endif
 
+// Retrieves the key used to sign metrics (UMA/UKM) uploads.
+std::string GetMetricsKey();
+
 // Represents the different sets of client IDs and secrets in use.
 enum OAuth2Client {
   CLIENT_MAIN,         // Several different features use this.