Reverts "[engine] always force platform channel responses to schedule a task. (#54975)" (#55000)

Reverts: #54975
Initiated by: jtmcdole
Reason for reverting: I believe this change caused flutter/flutter to break in an engine roll. It was one of two changes - the other being a webui change.

Tests broken:

module_test_ios:  testDualCold

router test:

```
[2024-09-05 17:43:20.837343] [STDOUT] stderr: [ +135 ms] VMServiceFlutterDriver: Connected to Flutter application.
[2024-09-05 17:43:20.841927] [STDOUT] stdout: [   +4 ms] 00:00 �[32m+0�[0m: 
Original PR Author: jonahwilliams

Reviewed By: {jason-simmons, johnmccutchan}

This change reverts the following previous change:
If we use runNowOrPostTask on platform channel responses, then we may not wake up the dart message loop. If nothing else wakes it up, then the embedder may hang on platform channel responses.

This fixes several google3 integration tests when run in merged thread mode.

Author: auto-submit[bot]

shell/common/shell.cc

@@ -1076,10 +1076,10 @@ void Shell::OnPlatformViewDispatchPlatformMessage(
 
   // The static leak checker gets confused by the use of fml::MakeCopyable.
   // NOLINTNEXTLINE(clang-analyzer-cplusplus.NewDeleteLeaks)
-  // This logic must always explicitly post a task so that we are guaranteed
-  // to wake up the UI message loop to flush tasks.
-  task_runners_.GetUITaskRunner()->PostTask(fml::MakeCopyable(
-      [engine = engine_->GetWeakPtr(), message = std::move(message)]() mutable {
+  fml::TaskRunner::RunNowOrPostTask(
+      task_runners_.GetUITaskRunner(),
+      fml::MakeCopyable([engine = engine_->GetWeakPtr(),
+                         message = std::move(message)]() mutable {
         if (engine) {
           engine->DispatchPlatformMessage(std::move(message));
         }