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/ImageDecodeBench.cpp b/bench/ImageDecodeBench.cpp
index a2ca0ec..47993cf 100644
--- a/bench/ImageDecodeBench.cpp
+++ b/bench/ImageDecodeBench.cpp
@@ -5,7 +5,7 @@
  * found in the LICENSE file.
  */
 
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkBitmap.h"
 #include "SkData.h"
 #include "SkForceLinking.h"
@@ -18,7 +18,7 @@
 
 class SkCanvas;
 
-class ImageDecodeBench : public SkBenchmark {
+class ImageDecodeBench : public Benchmark {
 public:
     ImageDecodeBench(void* p, const char* filename)
     : fName("image_decode_")
@@ -85,7 +85,7 @@
     SkMemoryStream  fStream;
     bool            fValid;
 
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };
 
 // These are files which call decodePalette