[fastcall] Support FastApiCallbackOptions::CreateForTesting

https://crrev.com/c/2817958 is going to support artificial
calls of NoAllocDirectCall for a testing purpose, and this
new API will be used there.

Change-Id: If47ba080eede96e91ba60b89ff502dd3d3e34b93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2822188Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73945}

include/v8-fast-api-calls.h

@@ -194,6 +194,8 @@
 
 namespace v8 {
 
+class Isolate;
+
 class CTypeInfo {
  public:
   enum class Type : uint8_t {
@@ -321,6 +323,14 @@ struct ApiObject {
  * \endcode
  */
 struct FastApiCallbackOptions {
+  /**
+   * Creates a new instance of FastApiCallbackOptions for testing purpose.  The
+   * returned instance may be filled with mock data.
+   */
+  static FastApiCallbackOptions CreateForTesting(Isolate* isolate) {
+    return {false, {0}};
+  }
+
   /**
    * If the callback wants to signal an error condition or to perform an
    * allocation, it must set options.fallback to true and do an early return