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/WriterBench.cpp b/bench/WriterBench.cpp
index 7696eab..906a9d2 100644
--- a/bench/WriterBench.cpp
+++ b/bench/WriterBench.cpp
@@ -6,11 +6,11 @@
  * found in the LICENSE file.
  */
 
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkCanvas.h"
 #include "SkWriter32.h"
 
-class WriterBench : public SkBenchmark {
+class WriterBench : public Benchmark {
 public:
     virtual bool isSuitableFor(Backend backend) SK_OVERRIDE {
         return backend == kNonRendering_Backend;
@@ -33,7 +33,7 @@
     }
 
 private:
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };
 
 ////////////////////////////////////////////////////////////////////////////////