rebaseline.py: fix --add-new when there are no expectations at all

tools/tests/rebaseline/output/add-new/output-expected/gm-expectations/Test-Mac10.6-MacMini4.1-GeForce320M-x86-Release/expected-results.json

Index: tools/tests/rebaseline/output/add-new/output-expected/gm-expectations/Test-Mac10.6-MacMini4.1-GeForce320M-x86-Release/expected-results.json
===================================================================
--- tools/tests/rebaseline/output/add-new/output-expected/gm-expectations/Test-Mac10.6-MacMini4.1-GeForce320M-x86-Release/expected-results.json	(revision 0)
+++ tools/tests/rebaseline/output/add-new/output-expected/gm-expectations/Test-Mac10.6-MacMini4.1-GeForce320M-x86-Release/expected-results.json	(working copy)
@@ -1,26 +1,36 @@
 {
-   "expected-results" : {
-      "aaclip_565.png" : {
-         "allowed-digests" : [
-            [ "bitmap-64bitMD5", 12345 ]
-         ],
-         "ignore-failure" : false
-      },
-      "aaclip_8888.png" : {
-         "allowed-digests" : [
-            [ "bitmap-64bitMD5", 67890 ]
-         ],
-         "ignore-failure" : false
-      },
-      "aaclip_gpu.png" : {
-         "allowed-digests" : [
-            [ "bitmap-64bitMD5", 11899819492385205974 ]
-         ],
-         "ignore-failure" : false
-      },
-      "aaclip_pdf.png" : {
-         "allowed-digests" : null,
-         "ignore-failure" : false
-      }
-   }
-}
+  "expected-results": {
+    "aaclip_565.png": {
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          6190901827590820995
+        ]
+      ]
+    }, 
+    "aaclip_8888.png": {
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          14456211900777561488
+        ]
+      ]
+    }, 
+    "aaclip_gpu.png": {
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          11899819492385205974
+        ]
+      ]
+    }, 
+    "aaclip_pdf.png": {
+      "allowed-digests": [
+        [
+          "bitmap-64bitMD5", 
+          13515207506164874564
+        ]
+      ]
+    }
+  }
+}