Second wave of Win64 warning cleanup

https://codereview.chromium.org/27343002/



git-svn-id: http://skia.googlecode.com/svn/trunk@11778 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/skdiff_utils.cpp b/tools/skdiff_utils.cpp
index 1f5289b..6f73a08 100644
--- a/tools/skdiff_utils.cpp
+++ b/tools/skdiff_utils.cpp
@@ -115,7 +115,7 @@
 static SkString filename_to_derived_filename(const SkString& filename, const char *suffix) {
     SkString diffName (filename);
     const char* cstring = diffName.c_str();
-    int dotOffset = strrchr(cstring, '.') - cstring;
+    size_t dotOffset = strrchr(cstring, '.') - cstring;
     diffName.remove(dotOffset, diffName.size() - dotOffset);
     diffName.append(suffix);