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/dm/DMBenchTask.cpp b/dm/DMBenchTask.cpp
index 619bbc1..7cd2fdc 100644
--- a/dm/DMBenchTask.cpp
+++ b/dm/DMBenchTask.cpp
@@ -41,18 +41,18 @@
     , fSampleCount(sampleCount) {}
 
 bool NonRenderingBenchTask::shouldSkip() const {
-    return !fBench->isSuitableFor(SkBenchmark::kNonRendering_Backend);
+    return !fBench->isSuitableFor(Benchmark::kNonRendering_Backend);
 }
 
 bool CpuBenchTask::shouldSkip() const {
-    return !fBench->isSuitableFor(SkBenchmark::kRaster_Backend);
+    return !fBench->isSuitableFor(Benchmark::kRaster_Backend);
 }
 
 bool GpuBenchTask::shouldSkip() const {
-    return kGPUDisabled || !fBench->isSuitableFor(SkBenchmark::kGPU_Backend);
+    return kGPUDisabled || !fBench->isSuitableFor(Benchmark::kGPU_Backend);
 }
 
-static void draw_raster(SkBenchmark* bench, SkColorType colorType) {
+static void draw_raster(Benchmark* bench, SkColorType colorType) {
     SkBitmap bitmap;
     AllocatePixels(colorType, bench->getSize().x(), bench->getSize().y(), &bitmap);
     SkCanvas canvas(bitmap);