[web snapshot] Support web snapshot magic when the source string is two byte

Bug: v8:11525
Change-Id: I28548c4eddcc7764be950950e16ac30b12ac8cdd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3297890Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78061}

src/codegen/compiler.cc

@@ -2804,7 +2804,8 @@ MaybeHandle<SharedFunctionInfo> GetSharedFunctionInfoForScriptImpl(
 
   if (V8_UNLIKELY(
           i::FLAG_experimental_web_snapshots &&
-          (source->IsExternalOneByteString() || source->IsSeqOneByteString()) &&
+          (source->IsExternalOneByteString() || source->IsSeqOneByteString() ||
+           source->IsExternalTwoByteString() || source->IsSeqTwoByteString()) &&
           source_length > 4)) {
     // Experimental: Treat the script as a web snapshot if it starts with the
     // magic byte sequence. TODO(v8:11525): Remove this once proper embedder

src/web-snapshot/web-snapshot.cc

@@ -973,6 +973,39 @@ bool WebSnapshotDeserializer::UseWebSnapshot(
     deserializer_.reset(
         new ValueDeserializer(isolate_, data_copy.get(), length));
     return Deserialize();
+  } else if (source->IsExternalTwoByteString()) {
+    // TODO(v8:11525): Implement end-to-end snapshot processing which gets rid
+    // of the need to copy the data here.
+    const v8::String::ExternalStringResource* resource =
+        ExternalTwoByteString::cast(*source).resource();
+    auto length = resource->length();
+    std::unique_ptr<uint8_t[]> data_copy(new uint8_t[length]);
+    {
+      DisallowGarbageCollection no_gc;
+      const uint16_t* data = resource->data();
+      uint8_t* data_copy_ptr = data_copy.get();
+      for (size_t i = 0; i < length; ++i) {
+        data_copy_ptr[i] = static_cast<uint8_t>(data[i]);
+      }
+    }
+    deserializer_.reset(
+        new ValueDeserializer(isolate_, data_copy.get(), length));
+    return Deserialize();
+  } else if (source->IsSeqTwoByteString()) {
+    SeqTwoByteString source_as_seq = SeqTwoByteString::cast(*source);
+    auto length = source_as_seq.length();
+    std::unique_ptr<uint8_t[]> data_copy(new uint8_t[length]);
+    {
+      DisallowGarbageCollection no_gc;
+      uint16_t* data = source_as_seq.GetChars(no_gc);
+      uint8_t* data_copy_ptr = data_copy.get();
+      for (int i = 0; i < length; ++i) {
+        data_copy_ptr[i] = static_cast<uint8_t>(data[i]);
+      }
+    }
+    deserializer_.reset(
+        new ValueDeserializer(isolate_, data_copy.get(), length));
+    return Deserialize();
   }
   UNREACHABLE();
 }