webmdec,WebmInputContext: make timestamp_ns signed

this matches the type returned from libwebm, which uses -1 as an error;
quiets integer sanitizer warnings of the form:
implicit conversion from type 'long long' of value -1 (64-bit, signed)
to type 'uint64_t' (aka 'unsigned long') changed the value to
18446744073709551615 (64-bit, unsigned)

Bug: b/229626362
Change-Id: Id3966912f802aee3c0f7852225b55f3057c3e76a

webmdec.h

@@ -27,7 +27,7 @@ struct WebmInputContext {
   const void *block;
   int block_frame_index;
   int video_track_index;
-  uint64_t timestamp_ns;
+  int64_t timestamp_ns;
   int is_key_frame;
   int reached_eos;
 };