Fix 64-bit truncate warning

BUG=chromium:910360

Change-Id: Ibd353d851626cca1b44bcfe38de5039a772bc6f0
Reviewed-on: https://skia-review.googlesource.com/c/178284
Auto-Submit: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/include/gpu/GrResourceKey.h b/include/gpu/GrResourceKey.h
index 006fae8..b599874 100644
--- a/include/gpu/GrResourceKey.h
+++ b/include/gpu/GrResourceKey.h
@@ -90,7 +90,7 @@
             SkDebugf("domain: %d ", this->domain());
             SkDebugf("size: %dB ", this->internalSize());
             for (size_t i = 0; i < this->internalSize(); ++i) {
-                SkDebugf("%d ", fKey[i]);
+                SkDebugf("%d ", fKey[SkTo<int>(i)]);
             }
             SkDebugf("\n");
         }