Demote getCount, getDepth, and clear to RTree-only methods.

We use them only to test RTree.

BUG=skia:

Review URL: https://codereview.chromium.org/622773003
diff --git a/bench/RTreeBench.cpp b/bench/RTreeBench.cpp
index 21c1d79..08d6bf4 100644
--- a/bench/RTreeBench.cpp
+++ b/bench/RTreeBench.cpp
@@ -24,7 +24,7 @@
 class RTreeBuildBench : public Benchmark {
 public:
     RTreeBuildBench(const char* name, MakeRectProc proc, bool bulkLoad,
-                    SkBBoxHierarchy* tree)
+                    SkRTree* tree)
         : fTree(tree)
         , fProc(proc)
         , fBulkLoad(bulkLoad) {
@@ -58,7 +58,7 @@
         }
     }
 private:
-    SkBBoxHierarchy* fTree;
+    SkRTree* fTree;
     MakeRectProc fProc;
     SkString fName;
     bool fBulkLoad;
@@ -76,7 +76,7 @@
     };
 
     RTreeQueryBench(const char* name, MakeRectProc proc, bool bulkLoad,
-                    QueryType q, SkBBoxHierarchy* tree)
+                    QueryType q, SkRTree* tree)
         : fTree(tree)
         , fProc(proc)
         , fBulkLoad(bulkLoad)