[wasm] Consider unknown sections as valid sections

This is just a minor refactoring: We include the kUnknownSectionCode in
the set of valid sections. There are only two uses of that function: The
first one already checks for kUnknownSectionCode in a separate
condition, the second one can be simplified with this refactoring.

R=ahaas@chromium.org

Bug: chromium:1335023
Change-Id: Ie1ae316db41f8111fff0af0d97a6c775f527156c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3698556
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81101}

src/wasm/module-decoder.h

@@ -29,7 +29,9 @@ namespace wasm {
 struct CompilationEnv;
 
 inline bool IsValidSectionCode(uint8_t byte) {
-  return kTypeSectionCode <= byte && byte <= kLastKnownModuleSection;
+  // Allow everything within [kUnknownSectionCode, kLastKnownModuleSection].
+  static_assert(kUnknownSectionCode == 0);
+  return byte <= kLastKnownModuleSection;
 }
 
 const char* SectionName(SectionCode code);

src/wasm/streaming-decoder.cc

@@ -588,9 +588,7 @@ std::unique_ptr<AsyncStreamingDecoder::DecodingState>
 AsyncStreamingDecoder::DecodeSectionID::Next(AsyncStreamingDecoder* streaming) {
   TRACE_STREAMING("DecodeSectionID: %u (%s)\n", id_,
                   SectionName(static_cast<SectionCode>(id_)));
-  if (id_ != kUnknownSectionCode && !IsValidSectionCode(id_)) {
-    return streaming->Error("invalid section code");
-  }
+  if (!IsValidSectionCode(id_)) return streaming->Error("invalid section code");
   if (id_ == SectionCode::kCodeSectionCode) {
     // Explicitly check for multiple code sections as module decoder never
     // sees the code section and hence cannot track this section.