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

gm/composeshader.cpp

Index: gm/composeshader.cpp
diff --git a/gm/composeshader.cpp b/gm/composeshader.cpp
index c029c9855959f0d2949c03212a76a7964d2aceab..b58f91f4c0599b12d3dcc8d9cc8ea1f9fa325911 100644
--- a/gm/composeshader.cpp
+++ b/gm/composeshader.cpp
@@ -210,7 +210,7 @@ private:
      *  work in a release build.  You can change this parameter and then compile a release build
      *  to have this GM draw larger bitmaps for easier visual inspection.
      */
-    static const int squareLength = 20;
+    static constexpr int squareLength = 20;
 
     SkBitmap fColorBitmap;
     SkBitmap fAlpha8Bitmap;