private iterator to visit all resource cache entries

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/1271033002
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 0a89b48..c985b1e 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -1473,9 +1473,18 @@
     return true;
 }
 
+#include "SkResourceCache.h"
+#include "SkGlyphCache.h"
 bool SampleWindow::nextSample() {
     fCurrIndex = (fCurrIndex + 1) % fSamples.count();
     this->loadView((*fSamples[fCurrIndex])());
+
+    if (false) {
+        SkResourceCache::TestDumpMemoryStatistics();
+        SkGlyphCache::Dump();
+        SkDebugf("\n");
+    }
+
     return true;
 }