[wasm][streaming] Report deterministic error location

This fixes a bug in the offset computation when instantiating the
decoder to decode a VarInt32.
It also extends the streaming decoder test to check the error location.

R=ahaas@chromium.org

Bug: v8:8814
Change-Id: Id8ce31ce7e494cce14231febbb5b0c7d91a26e01
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1505453Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60067}

src/wasm/streaming-decoder.cc

@@ -319,7 +319,8 @@ size_t StreamingDecoder::DecodeVarInt32::ReadBytes(
   TRACE_STREAMING("ReadBytes of a VarInt\n");
   memcpy(remaining_buf.start(), &bytes.first(), new_bytes);
   buf.Truncate(offset() + new_bytes);
-  Decoder decoder(buf, streaming->module_offset());
+  Decoder decoder(buf,
+                  streaming->module_offset() - static_cast<uint32_t>(offset()));
   value_ = decoder.consume_u32v(field_name_);
   // The number of bytes we actually needed to read.
   DCHECK_GT(decoder.pc(), buffer().start());