Remove Sk prefix from some bench classes.

This idea came while commenting on
https://codereview.chromium.org/343583005/

Since SkBenchmark, SkBenchLogger and SkGMBench are not part of the Skia library,
they should not have the Sk prefix.

BUG=None
TEST=make all
R=mtklein@google.com

Author: tfarina@chromium.org

Review URL: https://codereview.chromium.org/347823004
diff --git a/bench/MatrixBench.cpp b/bench/MatrixBench.cpp
index fa19fc8..78482d9 100644
--- a/bench/MatrixBench.cpp
+++ b/bench/MatrixBench.cpp
@@ -5,13 +5,13 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkMatrix.h"
 #include "SkMatrixUtils.h"
 #include "SkRandom.h"
 #include "SkString.h"
 
-class MatrixBench : public SkBenchmark {
+class MatrixBench : public Benchmark {
     SkString    fName;
 public:
     MatrixBench(const char name[])  {
@@ -38,7 +38,7 @@
     }
 
 private:
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };