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/GameBench.cpp b/bench/GameBench.cpp
index d197f3e..c37f46d 100644
--- a/bench/GameBench.cpp
+++ b/bench/GameBench.cpp
@@ -5,7 +5,7 @@
  * found in the LICENSE file.
  */
 
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkCanvas.h"
 #include "SkPaint.h"
 #include "SkRandom.h"
@@ -14,7 +14,7 @@
 
 // This bench simulates the calls Skia sees from various HTML5 canvas
 // game bench marks
-class GameBench : public SkBenchmark {
+class GameBench : public Benchmark {
 public:
     enum Type {
         kScale_Type,
@@ -304,7 +304,7 @@
         }
     }
 
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };
 
 // Partial clear