Add main task runner for Mojo message output

IME decoder shared library has its own threads. In order to make sure
all the child theads post message via mojo safely, we must post all of
these tasks to MainTaskRunner.

Bug: 837156
Change-Id: I564662bfae078844da47628ad95560bdc160e6a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868532
Commit-Queue: Leo Zhang <googleo@chromium.org>
Auto-Submit: Leo Zhang <googleo@chromium.org>
Reviewed-by: Marti Wong <martiw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707288}

chromeos/services/ime/ime_service.cc

@@ -33,7 +33,8 @@ enum SimpleDownloadError {
 }  // namespace
 
 ImeService::ImeService(mojo::PendingReceiver<mojom::ImeService> receiver)
-    : receiver_(this, std::move(receiver)) {
+    : receiver_(this, std::move(receiver)),
+      main_task_runner_(base::SequencedTaskRunnerHandle::Get()) {
   input_engine_ = chromeos::features::IsImeDecoderWithSandboxEnabled()
                       ? std::make_unique<DecoderEngine>(this)
                       : std::make_unique<InputEngine>();
@@ -77,7 +78,7 @@ const char* ImeService::GetImeBundleDir() {
 }
 
 const char* ImeService::GetImeGlobalDir() {
-  // Global IME data dir will not be supported yet.
+  // Global IME data is supported yet.
   return "";
 }
 
@@ -86,10 +87,11 @@ const char* ImeService::GetImeUserHomeDir() {
 }
 
 void ImeService::RunInMainSequence(ImeSequencedTask task, int task_id) {
-  // Always run tasks on current SequencedTaskRunner.
-  // It's necessary for making any call on a bound Mojo Remote.
-  base::SequencedTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE, base::BindOnce(task, task_id));
+  // This helps ensure that tasks run in the **main** SequencedTaskRunner.
+  // E.g. the Mojo Remotes are bound on the main_task_runner_, so that any task
+  // invoked Mojo call from other threads (sequences) should be posted to
+  // main_task_runner_ by this function.
+  main_task_runner_->PostTask(FROM_HERE, base::BindOnce(task, task_id));
 }
 
 int ImeService::SimpleDownloadToFile(const char* url,

chromeos/services/ime/ime_service.h

@@ -60,6 +60,7 @@ class ImeService : public mojom::ImeService,
                               const base::FilePath& file);
 
   mojo::Receiver<mojom::ImeService> receiver_;
+  scoped_refptr<base::SequencedTaskRunner> main_task_runner_;
 
   // For the duration of this service lifetime, there should be only one
   // input engine instance.