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/PicturePlaybackBench.cpp b/bench/PicturePlaybackBench.cpp
index 08c96d5..0109c72 100644
--- a/bench/PicturePlaybackBench.cpp
+++ b/bench/PicturePlaybackBench.cpp
@@ -4,7 +4,7 @@
  * 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 "SkCanvas.h"
 #include "SkColor.h"
 #include "SkPaint.h"
@@ -17,7 +17,7 @@
 // This is designed to emulate about 4 screens of textual content
 
 
-class PicturePlaybackBench : public SkBenchmark {
+class PicturePlaybackBench : public Benchmark {
 public:
     PicturePlaybackBench(const char name[])  {
         fName.printf("picture_playback_%s", name);
@@ -63,7 +63,7 @@
     SkScalar fPictureHeight;
     SkScalar fTextSize;
 private:
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };