unsigned -> int for counts and indices in picture-related code

also, (C)

BUG=skia:

Review URL: https://codereview.chromium.org/1300163002
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index 5b2ccc8..a7240b8 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -1060,7 +1060,7 @@
             macroCanvas,
             drawables ? *drawables : empty,
         };
-        for (unsigned i = 0; i < skr.count(); i++) {
+        for (int i = 0; i < skr.count(); i++) {
             skr.visit<void>(i, drawsAsSingletonPictures);
         }
         SkAutoTUnref<SkPicture> macroPic(macroRec.endRecordingAsPicture());