Set X-GoogleUpdate headers on Omaha requests.

When sending requests to Omaha, set the custom X-GoogleUpdate-* headers
to help identify the request type without parsing the actual request.

Bug: 28823234
TEST=Added unittests.

Change-Id: I4f0d64b4fc0f69e8a5d4b86337331a3a7a481b35
diff --git a/common/mock_http_fetcher.h b/common/mock_http_fetcher.h
index e56318e..367802e 100644
--- a/common/mock_http_fetcher.h
+++ b/common/mock_http_fetcher.h
@@ -91,6 +91,10 @@
   void SetHeader(const std::string& header_name,
                  const std::string& header_value) override;
 
+  // Return the value of the header |header_name| or the empty string if not
+  // set.
+  std::string GetHeader(const std::string& header_name) const;
+
   // Suspend the mock transfer.
   void Pause() override;