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

tests/DrawFilterTest.cpp

Index: tests/DrawFilterTest.cpp
diff --git a/tests/DrawFilterTest.cpp b/tests/DrawFilterTest.cpp
index d01fa07338cb7edc90431ab2aa75b3f96f4d6524..13ae5c6bd073753049a9b5b10115a71deefc4c73 100644
--- a/tests/DrawFilterTest.cpp
+++ b/tests/DrawFilterTest.cpp
@@ -10,12 +10,14 @@
 #include "SkSurface.h"
 #include "Test.h"
 
+namespace {
 class TestFilter : public SkDrawFilter {
 public:
     bool filter(SkPaint* p, Type) override {
         return true;
     }
 };
+}
 
 /**
  *  canvas.setDrawFilter is defined to be local to the save/restore block, such that if you