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

gm/gm.h

Index: gm/gm.h
===================================================================
--- gm/gm.h	(revision 9067)
+++ gm/gm.h	(working copy)
@@ -22,6 +22,11 @@
 
 namespace skiagm {
 
+    // TODO(epoger): Right now, various places in the code assume that all the
+    // image files read/written by GM use this file extension.
+    // Search for references to this constant to find these assumptions.
+    const static char kPNG_FileExtension[] = "png";
+
     static inline SkISize make_isize(int w, int h) {
         SkISize sz;
         sz.set(w, h);