Move the IconLoader to use the task scheduler.

chrome/browser/icon_loader.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/icon_loader.h"
 
+#include <utility>
+
 #include "base/bind.h"
+#include "base/task_scheduler/post_task.h"
+#include "base/task_scheduler/task_traits.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "content/public/browser/browser_thread.h"
 
 using content::BrowserThread;
 
 // static
 IconLoader* IconLoader::Create(const base::FilePath& file_path,
                                IconSize size,
                                IconLoadedCallback callback) {
   return new IconLoader(file_path, size, callback);
 }
 
 void IconLoader::Start() {
   target_task_runner_ = base::ThreadTaskRunnerHandle::Get();
 
-  BrowserThread::PostTaskAndReply(
-      BrowserThread::FILE, FROM_HERE,
+  base::PostTaskWithTraitsAndReply(
+      FROM_HERE, {base::MayBlock(), base::TaskPriority::USER_VISIBLE},
       base::BindOnce(&IconLoader::ReadGroup, base::Unretained(this)),
       base::BindOnce(&IconLoader::OnReadGroup, base::Unretained(this)));
 }
 
 IconLoader::IconLoader(const base::FilePath& file_path,
                        IconSize size,
                        IconLoadedCallback callback)
     : file_path_(file_path), icon_size_(size), callback_(callback) {}
 
 IconLoader::~IconLoader() {}
 
 void IconLoader::ReadGroup() {
   group_ = GroupForFilepath(file_path_);
 }
 
 void IconLoader::OnReadGroup() {
-  BrowserThread::PostTask(
-      ReadIconThreadID(), FROM_HERE,
-      base::BindOnce(&IconLoader::ReadIcon, base::Unretained(this)));
+  auto read_icon =
+      base::BindOnce(&IconLoader::ReadIcon, base::Unretained(this));
+  if (ReadIconRequiresUIThread()) {
+    content::BrowserThread::GetTaskRunnerForThread(content::BrowserThread::UI)
+        ->PostTask(FROM_HERE, std::move(read_icon));

[gab, 2017/06/22 15:38:35]

content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
std::move(read_icon));

+  } else {
+    base::PostTaskWithTraits(

[gab, 2017/06/22 15:38:35]

If you need COM you'll need to (on OS_WIN):

base::CreateCOMSTATaskRunnerWithTraits({base::MayBlock(),
base::TaskPriority::USER_VISIBLE})->PostTask(...);

(we unfortunately don't have a simpler one-off base::PostTask API for COM tasks
-- intentionally for now because this pattern doesn't come back that often and
the COM STA is inherently a single-threaded environment so it's nice to have to
request that explicitly)

Note: we're working on providing stricter COM checks to make sure such an
environment is requested (https://chromium-review.googlesource.com/c/537994/).
We currently allow COM everywhere in the TaskScheduler for backward
compatibility [1] but are looking to remove that as using an STA on a thread
that doesn't pump system messages is error-prone.

[1]
https://cs.chromium.org/chromium/src/base/task_scheduler/scheduler_worker.cc?...

+        FROM_HERE, {base::MayBlock(), base::TaskPriority::USER_VISIBLE},

[sky, 2017/06/22 00:43:25]

optional: create a function to return the traits that this and line 28 use.

[Avi (use Gerrit), 2017/06/22 01:21:17]

Ben: Is that the style for the new PostTask?

[gab, 2017/06/22 15:38:35]

TaskTraits is constexpr, you can store them in a common variable in the
anonymous namespace if you wish to re-use.

+        std::move(read_icon));
+  }
 }