Reland #2: Add a short term solution to fix OSX 10.11 system font rendering problems.

Source/platform/fonts/mac/FontFamilyMatcherMac.mm

  /*
  * Copyright (C) 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 115 matching lines @@
     // Otherwise, prefer the one closer to the desired weight.
     return candidateWeightDeltaMagnitude < chosenWeightDeltaMagnitude;
 }
 
 // Family name is somewhat of a misnomer here.  We first attempt to find an exact match
 // comparing the desiredFamily to the PostScript name of the installed fonts.  If that fails
 // we then do a search based on the family names of the installed fonts.
 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 (IsOSElCapitan()) {
+            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
         // tests don't support different expectations based on the SDK version,
         // so force layout tests to use "Lucida Grande". Once the 10.10 SDK
         // switch is made, this should be changed to return .LucidaGrandeUI and
         // the Layout Expectations should be updated. http://crbug.com/515836.
         if (LayoutTestSupport::isRunningLayoutTest() && IsOSMavericks()) {
             if (desiredWeight >= blink::FontWeightBold)
                 return [NSFont fontWithName:@"Lucida Grande Bold" size:size];
@@ skipping 140 matching lines @@
         6, // FontWeight500
         8, // FontWeight600
         9, // FontWeight700
         10, // FontWeight800
         12, // FontWeight900
     };
     return appKitFontWeights[fontWeight];
 }
 
 } // namespace blink