Move the IconLoader to use the task scheduler.

chrome/browser/icon_loader.cc

Index: chrome/browser/icon_loader.cc
diff --git a/chrome/browser/icon_loader.cc b/chrome/browser/icon_loader.cc
index d679bd3598331ad1bda2c5228218a4bef291b110..50cd8b68710d0f844be626cc954159bfda229027 100644
--- a/chrome/browser/icon_loader.cc
+++ b/chrome/browser/icon_loader.cc
@@ -4,7 +4,11 @@
 
 #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"
 
@@ -20,8 +24,8 @@ IconLoader* IconLoader::Create(const base::FilePath& file_path,
 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)));
 }
@@ -38,7 +42,14 @@ void IconLoader::ReadGroup() {
 }
 
 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));
+  }
 }