WIP: Allow ValueSerializer clients to transfer long strings out of band.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 38584e8af06efdd848ae92d5c8b8ad412108f528..c1d40b315012fff9c7ad1be6b8d7e7f816c8b0bd 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -1712,6 +1712,15 @@ class V8_EXPORT ValueSerializer {
 
     virtual Maybe<uint32_t> GetWasmModuleTransferId(
         Isolate* isolate, Local<WasmCompiledModule> module);
+
+    /*
+     * Called when the ValueSerializer encounters a string at least as long as
+     * the value passed to |SetMinimumLongStringLength|. The embedder is
+     * expected to return an ID for the string.
+     */
+    virtual Maybe<uint32_t> GetLongStringId(Isolate* isolate,
+                                            Local<String> string);
+
     /*
      * Allocates memory for the buffer of at least the size provided. The actual
      * size (which may be greater or equal) is written to |actual_size|. If no
@@ -1785,6 +1794,14 @@ class V8_EXPORT ValueSerializer {
   void SetTreatArrayBufferViewsAsHostObjects(bool mode);
 
   /*
+   * If a positive values is assigned, strings of at least this length will be
+   * passed to the delegate instead of serialized inline into the buffer.
+   * This option is recommended only if the delegate has a more efficient means
+   * of delivering those strings. By default, this feature is disabled.
+   */
+  void SetMinimumLongStringLength(int size);
+
+  /*
    * Write raw data in various common formats to the buffer.
    * Note that integer types are written in base-128 varint format, not with a
    * binary copy. For use during an override of Delegate::WriteHostObject.
@@ -1863,6 +1880,12 @@ class V8_EXPORT ValueDeserializer {
                                  Local<SharedArrayBuffer> shared_array_buffer);
 
   /*
+   * Similar to TransferArrayBuffer, but for IDs obtained from
+   * |GetLongStringId|.
+   */
+  void TransferLongString(uint32_t id, Local<String> string);
+
+  /*
    * Must be called before ReadHeader to enable support for reading the legacy
    * wire format (i.e., which predates this being shipped).
    *