When using clang, switch on -Wc++11-narrowing

See
https://clang.llvm.org/docs/DiagnosticsReference.html#wc-11-narrowing
for datails. This catches a narrowing bug that broke a downstream
project in https://codereview.webrtc.org/2995523002/.

BUG=none

Review-Url: https://codereview.webrtc.org/2995073002
Cr-Commit-Position: refs/heads/master@{#19366}
diff --git a/webrtc/modules/video_coding/frame_buffer2_unittest.cc b/webrtc/modules/video_coding/frame_buffer2_unittest.cc
index 95a879b..88f8410 100644
--- a/webrtc/modules/video_coding/frame_buffer2_unittest.cc
+++ b/webrtc/modules/video_coding/frame_buffer2_unittest.cc
@@ -153,7 +153,8 @@
                   T... refs) {
     static_assert(sizeof...(refs) <= kMaxReferences,
                   "To many references specified for FrameObject.");
-    std::array<uint16_t, sizeof...(refs)> references = {{refs...}};
+    std::array<uint16_t, sizeof...(refs)> references = {
+        {rtc::checked_cast<uint16_t>(refs)...}};
 
     std::unique_ptr<FrameObjectFake> frame(new FrameObjectFake());
     frame->picture_id = picture_id;