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/MipMapBench.cpp b/bench/MipMapBench.cpp
index 2fb52bb..137fbe4 100644
--- a/bench/MipMapBench.cpp
+++ b/bench/MipMapBench.cpp
@@ -27,7 +27,7 @@
         fBitmap.eraseColor(SK_ColorWHITE);  // so we don't read uninitialized memory
     }
 
-    void onDraw(const int loops, SkCanvas*) override {
+    void onDraw(int loops, SkCanvas*) override {
         for (int i = 0; i < loops; i++) {
             SkMipMap::Build(fBitmap, nullptr)->unref();
         }