Add a short term solution to fix OSX 10.11 system font rendering problems.

Source/platform/fonts/mac/FontFamilyMatcherMac.mm

Index: Source/platform/fonts/mac/FontFamilyMatcherMac.mm
diff --git a/Source/platform/fonts/mac/FontFamilyMatcherMac.mm b/Source/platform/fonts/mac/FontFamilyMatcherMac.mm
index d71d4b223a1fff0288e7b4ddaca238a49ca1f2a5..3000356ef695ff943f7cf44d094ac02f3518ebcb 100644
--- a/Source/platform/fonts/mac/FontFamilyMatcherMac.mm
+++ b/Source/platform/fonts/mac/FontFamilyMatcherMac.mm
@@ -133,6 +133,15 @@ static BOOL betterChoice(NSFontTraitMask desiredTraits, int desiredWeight,
 NSFont* MatchNSFontFamily(NSString* desiredFamily, NSFontTraitMask desiredTraits, FontWeight desiredWeight, float size)
 {
     if ([desiredFamily isEqualToString:@"BlinkMacSystemFont"]) {
+        // On OSX 10.11, returning the default system font causes rendering
+        // problems. In the short term, return nil, which is what would be
+        // returned if "BlinkMacSystemFont" was never introduced.
+        // TODO(erikchen): Fix the rendering problems.
+        // http://crbug.com/521034.
+        if (!IsOSYosemiteOrEarlier()) {
+            return nil;
+        }
+
         // On OSX 10.9, the default system font depends on the SDK version. When
         // compiled against the OSX 10.10 SDK, the font is .LucidaGrandeUI. When
         // compiled against the OSX 10.6 SDK, the font is Lucida Grande. Layout