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

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/1079813002

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;

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