GM: stop recording all *_pdf.png results as "INVALID" on non-Mac platforms

gm/tests/outputs/add-config-pdf/output-expected/json-summary.txt

Index: gm/tests/outputs/add-config-pdf/output-expected/json-summary.txt
===================================================================
--- gm/tests/outputs/add-config-pdf/output-expected/json-summary.txt	(revision 0)
+++ gm/tests/outputs/add-config-pdf/output-expected/json-summary.txt	(revision 0)
@@ -0,0 +1,25 @@
+{
+   "actual-results" : {
+      "failed" : null,
+      "failure-ignored" : null,
+      "no-comparison" : null,

[epoger, 2013/06/18 05:20:10]

The JSON summary looks better now.  No INVALID, just no record of the pdf
rendering (which is fine, because we can't compare PDFs on this platform)

+      "succeeded" : {
+         "565/selftest1.png" : [ "bitmap-64bitMD5", 12927999507540085554 ],
+         "8888/selftest1.png" : [ "bitmap-64bitMD5", 1209453360120438698 ]
+      }
+   },
+   "expected-results" : {
+      "565/selftest1.png" : {
+         "allowed-digests" : [
+            [ "bitmap-64bitMD5", 12927999507540085554 ]
+         ],
+         "ignore-failure" : false
+      },
+      "8888/selftest1.png" : {
+         "allowed-digests" : [
+            [ "bitmap-64bitMD5", 1209453360120438698 ]
+         ],
+         "ignore-failure" : false
+      }
+   }
+}