Consider platform-specific expected .png files.

testing/tools/pngdiffer.py

Index: testing/tools/pngdiffer.py
diff --git a/testing/tools/pngdiffer.py b/testing/tools/pngdiffer.py
index dc65b4717f428a12f204d69afb3dd98cd3b1f915..7a7e8ddbc63f4590aacc7f18d7a0fb6af37226ff 100755
--- a/testing/tools/pngdiffer.py
+++ b/testing/tools/pngdiffer.py
@@ -10,9 +10,11 @@ import sys
 class PNGDiffer():
   ACTUAL_TEMPLATE = '.pdf.%d.png'
   EXPECTED_TEMPLATE = '_expected' + ACTUAL_TEMPLATE
+  PLATFORM_EXPECTED_TEMPLATE = '_expected_%s' + ACTUAL_TEMPLATE
 
   def __init__(self, finder):
     self.pdfium_diff_path = finder.ExecutablePath('pdfium_diff')
+    self.os_name = finder.os_name
 
   def HasDifferences(self, input_filename, source_dir, working_dir):
     input_root, _ = os.path.splitext(input_filename)
@@ -20,12 +22,18 @@ class PNGDiffer():
         working_dir, input_root + self.ACTUAL_TEMPLATE)
     expected_path_template = os.path.join(
         source_dir, input_root + self.EXPECTED_TEMPLATE)
+    platform_expected_path_template = os.path.join(
+        source_dir, input_root + self.PLATFORM_EXPECTED_TEMPLATE)
     i = 0
     try:
       while True:
-        actual_path = actual_path_template % i;
-        expected_path = expected_path_template % i;
-        if not os.path.exists(expected_path):
+        actual_path = actual_path_template % i
+        expected_path = expected_path_template % i
+        platform_expected_path = (
+            platform_expected_path_template % (self.os_name, i))
+        if os.path.exists(platform_expected_path):
+          expected_path = platform_expected_path
+        elif not os.path.exists(expected_path):
           if i == 0:
             print "WARNING: no expected results files for " + input_filename
           break