[wasm] Add a flag to enable/disable async compile.

Adding a flag to explicitly enable/disable async compile.
Will cherrypick this to M59.
Separately will land a change to re-enable this by default.

BUG=v8:6264
R=mtrofin@chromium.org,ahaas@chromium.org,hablich@chromium.org

Review-Url: https://codereview.chromium.org/2820553003
Cr-Commit-Position: refs/heads/master@{#44653}

src/flag-definitions.h

@@ -534,6 +534,8 @@ DEFINE_BOOL(wasm_disable_structured_cloning, false,
             "disable WASM structured cloning")
 DEFINE_INT(wasm_num_compilation_tasks, 10,
            "number of parallel compilation tasks for wasm")
+DEFINE_BOOL(wasm_async_compilation, false,
+            "enable actual asynchronous compilation for WebAssembly.compile")
 // Parallel compilation confuses turbo_stats, force single threaded.
 DEFINE_VALUE_IMPLICATION(turbo_stats, wasm_num_compilation_tasks, 0)
 DEFINE_UINT(wasm_max_mem_pages, v8::internal::wasm::kV8MaxWasmMemoryPages,

src/wasm/wasm-module.cc

@@ -3038,6 +3038,20 @@ class AsyncCompileJob {
 
 void wasm::AsyncCompile(Isolate* isolate, Handle<JSPromise> promise,
                         const ModuleWireBytes& bytes) {
+  if (!FLAG_wasm_async_compilation) {
+    ErrorThrower thrower(isolate, "WasmCompile");
+    // Compile the module.
+    MaybeHandle<WasmModuleObject> module_object =
+        SyncCompile(isolate, &thrower, bytes);
+    if (thrower.error()) {
+      RejectPromise(isolate, handle(isolate->context()), &thrower, promise);
+      return;
+    }
+    Handle<WasmModuleObject> module = module_object.ToHandleChecked();
+    ResolvePromise(isolate, handle(isolate->context()), promise, module);
+    return;
+  }
+
   // Make a copy of the wire bytes in case the user program changes them
   // during asynchronous compilation.
   std::unique_ptr<byte[]> copy(new byte[bytes.length()]);