DM warning-free on win64

Review URL: https://codereview.chromium.org/805543002
diff --git a/tests/SkBase64Test.cpp b/tests/SkBase64Test.cpp
index 14ff1f7..a534768 100644
--- a/tests/SkBase64Test.cpp
+++ b/tests/SkBase64Test.cpp
@@ -20,7 +20,7 @@
         size_t encodeLength = SkBase64::Encode(all + offset, length, NULL);
         SkAutoTMalloc<char> src(encodeLength + 1);
         SkBase64::Encode(all + offset, length, src.get());
-        src[encodeLength] = '\0';
+        src[SkToInt(encodeLength)] = '\0';
         SkBase64 tryMe;
         tryMe.decode(src.get(), encodeLength);
         REPORTER_ASSERT(reporter, (strcmp((const char*) (all + offset), tryMe.getData()) == 0));