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/ReadPixBench.cpp b/bench/ReadPixBench.cpp
index 805aaeb..7848ebc 100644
--- a/bench/ReadPixBench.cpp
+++ b/bench/ReadPixBench.cpp
@@ -6,7 +6,7 @@
  * found in the LICENSE file.
  */
 
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkCanvas.h"
 
 
@@ -15,7 +15,7 @@
  * and then reads small chunks from it repeatedly. This can cause trouble
  * for the GPU as readbacks are very expensive.
  */
-class ReadPixBench : public SkBenchmark {
+class ReadPixBench : public Benchmark {
 public:
     ReadPixBench() {}
 
@@ -59,7 +59,7 @@
     static const int kNumStepsY = 30;
     static const int kWindowSize = 5;
 
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };
 
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