Add ability to output ImageBaseGSUrl to render_picture and use in rebaseline server

tools/image_expectations.h

Index: tools/image_expectations.h
diff --git a/tools/image_expectations.h b/tools/image_expectations.h
index 2d58b929737fe6feebfceb79adf98be48b75fdc0..422c64d4eda78b379c6fa6495fd941fdc084ac2d 100644
--- a/tools/image_expectations.h
+++ b/tools/image_expectations.h
@@ -185,6 +185,13 @@ namespace sk_tools {
         void addDescription(const char *key, const char *value);
 
         /**
+         * Adds the image base Google Storage URL to the summary of results.
+         *
+         * @param imageBaseGSUrl the image base Google Storage URL
+         */
+        void setImageBaseGSUrl(const char *imageBaseGSUrl);
+
+        /**
          * Returns the Expectation for this test.
          *
          * @param sourceName name of the source file that generated this result
@@ -217,6 +224,7 @@ namespace sk_tools {
         Json::Value fDescriptions;
         Json::Value fExpectedJsonRoot;
         Json::Value fExpectedResults;
+        Json::Value fImageBaseGSUrl;
     };
 
 } // namespace sk_tools