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/GradientBench.cpp b/bench/GradientBench.cpp
index fa07bce..90d45c8 100644
--- a/bench/GradientBench.cpp
+++ b/bench/GradientBench.cpp
@@ -5,7 +5,7 @@
  * 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 "SkBitmap.h"
 #include "SkCanvas.h"
 #include "SkColorPriv.h"
@@ -197,7 +197,7 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
-class GradientBench : public SkBenchmark {
+class GradientBench : public Benchmark {
     SkString fName;
     SkShader* fShader;
     enum {
@@ -262,7 +262,7 @@
     }
 
 private:
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 
     GeomType fGeomType;
 };
@@ -306,7 +306,7 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 
-class Gradient2Bench : public SkBenchmark {
+class Gradient2Bench : public Benchmark {
     SkString fName;
     bool     fHasAlpha;
 
@@ -347,7 +347,7 @@
     }
 
 private:
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };
 
 DEF_BENCH( return new Gradient2Bench(false); )