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/StrokeBench.cpp b/bench/StrokeBench.cpp
index 975c01f..24e0cab 100644
--- a/bench/StrokeBench.cpp
+++ b/bench/StrokeBench.cpp
@@ -5,7 +5,7 @@
  * found in the LICENSE file.
  */
 
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkCanvas.h"
 #include "SkPaint.h"
 #include "SkRRect.h"
@@ -53,7 +53,7 @@
 // Test drawing a small stroked version to see the effect of special-casing
 // our stroke code for these convex single-contour shapes.
 //
-class StrokeRRectBench : public SkBenchmark {
+class StrokeRRectBench : public Benchmark {
     SkString fName;
     SkPaint::Join fJoin;
     RRectRec fRec;
@@ -88,7 +88,7 @@
     }
 
 private:
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };
 
 DEF_BENCH( return new StrokeRRectBench(SkPaint::kRound_Join, draw_rect); )