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/PremulAndUnpremulAlphaOpsBench.cpp b/bench/PremulAndUnpremulAlphaOpsBench.cpp
index e937e14..311bdf2 100644
--- a/bench/PremulAndUnpremulAlphaOpsBench.cpp
+++ b/bench/PremulAndUnpremulAlphaOpsBench.cpp
@@ -5,13 +5,13 @@
  * found in the LICENSE file.
  */
 
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkCanvas.h"
 #include "SkConfig8888.h"
 #include "SkString.h"
 #include "sk_tool_utils.h"
 
-class PremulAndUnpremulAlphaOpsBench : public SkBenchmark {
+class PremulAndUnpremulAlphaOpsBench : public Benchmark {
     enum {
         W = 256,
         H = 256,
@@ -58,7 +58,7 @@
     SkColorType fColorType;
     SkString fName;
 
-    typedef SkBenchmark INHERITED;
+    typedef Benchmark INHERITED;
 };