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

gm/drawatlascolor.cpp

Index: gm/drawatlascolor.cpp
diff --git a/gm/drawatlascolor.cpp b/gm/drawatlascolor.cpp
index 333148253884eff316ff65144d8f66b62175764c..b5dba99640bd1a5bed9f732ec484258a1085e7fa 100644
--- a/gm/drawatlascolor.cpp
+++ b/gm/drawatlascolor.cpp
@@ -162,11 +162,11 @@ protected:
     }
 
 private:
-    static const int kNumXferModes = 29;
-    static const int kNumColors = 4;
-    static const int kAtlasSize = 30;
-    static const int kPad = 2;
-    static const int kTextPad = 8;
+    static constexpr int kNumXferModes = 29;
+    static constexpr int kNumColors = 4;
+    static constexpr int kAtlasSize = 30;
+    static constexpr int kPad = 2;
+    static constexpr int kTextPad = 8;
 
 
     sk_sp<SkImage> fAtlas;