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/StackBench.cpp b/bench/StackBench.cpp
index 61af99f..3b41cb6 100644
--- a/bench/StackBench.cpp
+++ b/bench/StackBench.cpp
@@ -5,7 +5,7 @@
  * found in the LICENSE file.
  */
 
-#include "SkBenchmark.h"
+#include "Benchmark.h"
 #include "SkRandom.h"
 
 #include "SkChunkAlloc.h"
@@ -29,7 +29,7 @@
 // It wins every benchmark on every machine I tried (Desktop, Nexus S, Laptop).
 
 template <typename Impl>
-struct StackBench : public SkBenchmark {
+struct StackBench : public Benchmark {
     virtual bool isSuitableFor(Backend b) SK_OVERRIDE { return b == kNonRendering_Backend; }
     virtual const char* onGetName() SK_OVERRIDE { return Impl::kName; }
     virtual void onDraw(const int loops, SkCanvas*) SK_OVERRIDE { Impl::bench(loops); }