Bench : Unused parameters cleanup

I removed unused parameters in bench wherever it was trivial to do so.
Review URL: https://codereview.appspot.com/7411046

git-svn-id: http://skia.googlecode.com/svn/trunk@7988 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/bench/MutexBench.cpp b/bench/MutexBench.cpp
index 6c152a0..d96f3f6 100644
--- a/bench/MutexBench.cpp
+++ b/bench/MutexBench.cpp
@@ -21,7 +21,7 @@
         return "mutex";
     }
 
-    virtual void onDraw(SkCanvas* canvas) {
+    virtual void onDraw(SkCanvas*) {
         for (int i = 0; i < N; i++) {
             SK_DECLARE_STATIC_MUTEX(mu);
             for (int j = 0; j < M; j++) {