use anon namespace to avoid (silent) collisions between local test classes

gm/drawfilter.cpp

Index: gm/drawfilter.cpp
diff --git a/gm/drawfilter.cpp b/gm/drawfilter.cpp
index 40323c9ac8ad7975e3f602c0f0acbd06e8809f9b..8c29f07b0268e8a2d34f6f9c34f097e952ca1806 100644
--- a/gm/drawfilter.cpp
+++ b/gm/drawfilter.cpp
@@ -19,6 +19,7 @@
  * the second red and sharp.
  */
 
+namespace {
 class TestFilter : public SkDrawFilter {
 public:
     bool filter(SkPaint* p, Type) override {
@@ -27,6 +28,7 @@ public:
         return true;
     }
 };
+}
 
 class DrawFilterGM : public skiagm::GM {
     SkAutoTUnref<SkMaskFilter> fBlur;