C++11: Replace 0 with nullptr where applicable in layout code.

Source/core/layout/ImageQualityController.cpp

Index: Source/core/layout/ImageQualityController.cpp
diff --git a/Source/core/layout/ImageQualityController.cpp b/Source/core/layout/ImageQualityController.cpp
index 41cf5bd7087d228a5e10ad0b33b1a6937cdcad24..7795bd090c6f84596cce55ec2e01a4f71539326e 100644
--- a/Source/core/layout/ImageQualityController.cpp
+++ b/Source/core/layout/ImageQualityController.cpp
@@ -39,7 +39,7 @@ namespace blink {
 
 static const double cLowQualityTimeThreshold = 0.500; // 500 ms
 
-static ImageQualityController* gImageQualityController = 0;
+static ImageQualityController* gImageQualityController = nullptr;
 
 ImageQualityController* ImageQualityController::imageQualityController()
 {
@@ -55,7 +55,7 @@ void ImageQualityController::remove(LayoutObject* layoutObject)
         gImageQualityController->objectDestroyed(layoutObject);
         if (gImageQualityController->isEmpty()) {
             delete gImageQualityController;
-            gImageQualityController = 0;
+            gImageQualityController = nullptr;
         }
     }
 }