Replace a lot of 'static const' with 'constexpr' or 'const'.

gm/gammacolorfilter.cpp

Index: gm/gammacolorfilter.cpp
diff --git a/gm/gammacolorfilter.cpp b/gm/gammacolorfilter.cpp
index ecb2daa2147a50f289f53bdad896fe343e7c3f66..f5e6542062a335b22bec140ddfcba0e05bb19167 100644
--- a/gm/gammacolorfilter.cpp
+++ b/gm/gammacolorfilter.cpp
@@ -88,9 +88,9 @@ protected:
     }
 
 private:
-    static const int kCellWidth = 64;
-    static const int kCellHeight = 64;
-    static const int kNumGreySteps = 16;
+    static constexpr int kCellWidth = 64;
+    static constexpr int kCellHeight = 64;
+    static constexpr int kNumGreySteps = 16;
 
     typedef GM INHERITED;
 };