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/InterpBench.cpp b/bench/InterpBench.cpp
index f6f70d4..b19af8f 100644
--- a/bench/InterpBench.cpp
+++ b/bench/InterpBench.cpp
@@ -1,13 +1,13 @@
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkColorPriv.h"
 #include "SkMatrix.h"
+#include "SkPaint.h"
 #include "SkRandom.h"
 #include "SkString.h"
-#include "SkPaint.h"
 
 #define TILE(x, width)  (((x) & 0xFFFF) * width >> 16)
 
-class InterpBench : public SkBenchmark {
+class InterpBench : public Benchmark {
     enum {
         kBuffer = 128,
         kLoop   = 20000
@@ -43,7 +43,7 @@
     }
 
 private:
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };
 
 class Fixed16D16Interp : public InterpBench {