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/ColorPrivBench.cpp b/bench/ColorPrivBench.cpp
index 9c15a84..01810ed 100644
--- a/bench/ColorPrivBench.cpp
+++ b/bench/ColorPrivBench.cpp
@@ -1,10 +1,10 @@
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkColorPriv.h"
 #include "SkRandom.h"
 #include "SkString.h"
 
 template <bool kFast, bool kScale>
-class FourByteInterpBench : public SkBenchmark {
+class FourByteInterpBench : public Benchmark {
 public:
     FourByteInterpBench() {
         fName.set("four_byte_interp");