StringImpl should not be referred from StringImplCF.

Source/core/rendering/RenderThemeChromiumMac.mm

Index: Source/core/rendering/RenderThemeChromiumMac.mm
diff --git a/Source/core/rendering/RenderThemeChromiumMac.mm b/Source/core/rendering/RenderThemeChromiumMac.mm
index 22f7d186d94988c944d994e8c1faec7a424128ed..aef7a5d053bf52496ef0c5ec8a4884d7637809e2 100644
--- a/Source/core/rendering/RenderThemeChromiumMac.mm
+++ b/Source/core/rendering/RenderThemeChromiumMac.mm
@@ -303,7 +303,8 @@ void RenderThemeChromiumMac::systemFont(CSSValueID cssValueId, FontDescription&
         NSFontManager *fontManager = [NSFontManager sharedFontManager];
         cachedDesc->setIsAbsoluteSize(true);
         cachedDesc->setGenericFamily(FontDescription::NoFamily);
-        cachedDesc->firstFamily().setFamily([font webCoreFamilyName]);
+        NSString* nsFamilyName = [font webCoreFamilyName];
+        cachedDesc->firstFamily().setFamily(AtomicString([nsFamilyName UTF8String]));
         cachedDesc->setSpecifiedSize([font pointSize]);
         cachedDesc->setWeight(toFontWeight([fontManager weightOfFont:font]));
         cachedDesc->setItalic([fontManager traitsOfFont:font] & NSItalicFontMask);
@@ -725,7 +726,8 @@ void RenderThemeChromiumMac::setFontFromControlSize(RenderStyle* style, NSContro
     fontDescription.setGenericFamily(FontDescription::SerifFamily);
 
     NSFont* font = [NSFont systemFontOfSize:[NSFont systemFontSizeForControlSize:controlSize]];
-    fontDescription.firstFamily().setFamily([font webCoreFamilyName]);
+    NSString* nsFamilyName = [font webCoreFamilyName];
+    fontDescription.firstFamily().setFamily(AtomicString([nsFamilyName UTF8String]));
     fontDescription.setComputedSize([font pointSize] * style->effectiveZoom());
     fontDescription.setSpecifiedSize([font pointSize] * style->effectiveZoom());