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

gm/bigtileimagefilter.cpp

Index: gm/bigtileimagefilter.cpp
diff --git a/gm/bigtileimagefilter.cpp b/gm/bigtileimagefilter.cpp
index b74f62ce61d684ec24df0dc0f02845472a50f6ad..429ef1e877faf75c622ea0c9db808da4035b9a84 100644
--- a/gm/bigtileimagefilter.cpp
+++ b/gm/bigtileimagefilter.cpp
@@ -94,9 +94,9 @@ protected:
     }
 
 private:
-    static const int kWidth = 512;
-    static const int kHeight = 512;
-    static const int kBitmapSize = 64;
+    static constexpr int kWidth = 512;
+    static constexpr int kHeight = 512;
+    static constexpr int kBitmapSize = 64;
 
     sk_sp<SkImage> fRedImage;
     sk_sp<SkImage> fGreenImage;