GM: include filename extension (.png) of each output file in JSON summary

gm/gm_expectations.h

Index: gm/gm_expectations.h
===================================================================
--- gm/gm_expectations.h	(revision 9118)
+++ gm/gm_expectations.h	(working copy)
@@ -41,6 +41,16 @@
 
     void gm_fprintf(FILE *stream, const char format[], ...);
 
+    /**
+     * Assembles rootPath and relativePath into a single path, like this:
+     * rootPath/relativePath
+     *
+     * Uses SkPATH_SEPARATOR, to work on all platforms.
+     *
+     * TODO(epoger): This should probably move into SkOSFile.h
+     */
+    SkString SkPathJoin(const char *rootPath, const char *relativePath);
+
     SkString make_filename(const char path[],
                            const char renderModeDescriptor[],
                            const char *name,