[compiler] Update StringRef::ToNumber for an external strings case

ToNumber was already returning base::Optional but it still needed to be
updated for the internal external uncached string case.

As a note, both IsExternal and IsSeqString do not need to be updated
since they only look at the map.

Bug: v8:7790
Change-Id: Icb5ba7f40982c01cada2a9c2b96b824edce70d44
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2615422Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72005}

src/compiler/js-heap-broker.cc

@@ -3102,6 +3102,14 @@ base::Optional<uint16_t> StringRef::GetFirstChar() {
 
 base::Optional<double> StringRef::ToNumber() {
   if (data_->should_access_heap()) {
+    if (data_->kind() == kNeverSerializedHeapObject &&
+        !this->IsInternalizedString()) {
+      TRACE_BROKER_MISSING(
+          broker(),
+          "number for kNeverSerialized non-internalized string " << *this);
+      return base::nullopt;
+    }
+
     return TryStringToDouble(broker()->local_isolate(), object());
   }
   return data()->AsString()->to_number();