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/WritePixelsBench.cpp b/bench/WritePixelsBench.cpp
index 80aeda7..5138375 100644
--- a/bench/WritePixelsBench.cpp
+++ b/bench/WritePixelsBench.cpp
@@ -5,11 +5,11 @@
  * found in the LICENSE file.
  */
 
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkCanvas.h"
 #include "SkString.h"
 
-class WritePixelsBench : public SkBenchmark {
+class WritePixelsBench : public Benchmark {
 public:
     WritePixelsBench(SkColorType ct, SkAlphaType at)
         : fColorType(ct)
@@ -68,7 +68,7 @@
     SkAlphaType fAlphaType;
     SkString    fName;
 
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };
 
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