Remove const from `const int loops`.

This drives me nuts, and prevents `while (loops --> 0)`.

BUG=skia:

Review URL: https://codereview.chromium.org/1379923005
diff --git a/bench/ReadPixBench.cpp b/bench/ReadPixBench.cpp
index 917b917..7df5260 100644
--- a/bench/ReadPixBench.cpp
+++ b/bench/ReadPixBench.cpp
@@ -24,7 +24,7 @@
         return "readpix";
     }
 
-    void onDraw(const int loops, SkCanvas* canvas) override {
+    void onDraw(int loops, SkCanvas* canvas) override {
         canvas->clear(SK_ColorBLACK);
 
         SkISize size = canvas->getDeviceSize();