update_engine: Attach session ID to HTTP header of binary download

In order for Omaha to correlate Omaha Client requests with the actual
binary download, the session ID must be attached to the HTTP header of
the binary download in the X-Goog-Update-SessionId.

Also, remove the HTTP header of X-Goog-Update-SessionId added into the
Omaha requests.

BUG=chromium:940515
TEST=unittests # new unittests

Change-Id: I0759562f2d1c8c003064ad976ca1ae6ce039b960
diff --git a/common/mock_http_fetcher.h b/common/mock_http_fetcher.h
index 492e6ce..0f04319 100644
--- a/common/mock_http_fetcher.h
+++ b/common/mock_http_fetcher.h
@@ -89,6 +89,12 @@
   void SetHeader(const std::string& header_name,
                  const std::string& header_value) override;
 
+  bool GetHeader(const std::string& header_name,
+                 std::string* header_value) const override {
+    header_value->clear();
+    return false;
+  }
+
   // Return the value of the header |header_name| or the empty string if not
   // set.
   std::string GetHeader(const std::string& header_name) const;