Even more win64 warning fixes

Review URL: https://codereview.chromium.org/800993002
diff --git a/tests/ImageCacheTest.cpp b/tests/ImageCacheTest.cpp
index eaf3f28..c34f9d9 100644
--- a/tests/ImageCacheTest.cpp
+++ b/tests/ImageCacheTest.cpp
@@ -56,7 +56,7 @@
 
     if (testPurge) {
         // stress test, should trigger purges
-        for (size_t i = 0; i < COUNT * 100; ++i) {
+        for (int i = 0; i < COUNT * 100; ++i) {
             TestingKey key(i);
             cache.add(SkNEW_ARGS(TestingRec, (key, i)));
         }