Remove the usage of BrowserThread::FILE in the shell_integration* files

chrome/browser/shell_integration.cc

Index: chrome/browser/shell_integration.cc
diff --git a/chrome/browser/shell_integration.cc b/chrome/browser/shell_integration.cc
index a846887aa0d8bb3a30a26c9878843fec0096d1bd..36881ea6375dad3da16ddad7be006a9ab2022f60 100644
--- a/chrome/browser/shell_integration.cc
+++ b/chrome/browser/shell_integration.cc
@@ -11,6 +11,7 @@
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread_restrictions.h"
 #include "build/build_config.h"
 #include "chrome/browser/policy/policy_path_parser.h"
@@ -136,15 +137,14 @@ base::string16 GetAppShortcutsSubdirName() {
 //
 
 void DefaultWebClientWorker::StartCheckIsDefault() {
-  BrowserThread::PostTask(
-      BrowserThread::FILE, FROM_HERE,
+  GetTaskRunner()->PostTask(
+      FROM_HERE,
       base::Bind(&DefaultWebClientWorker::CheckIsDefault, this, false));
 }
 
 void DefaultWebClientWorker::StartSetAsDefault() {
-  BrowserThread::PostTask(
-      BrowserThread::FILE, FROM_HERE,
-      base::Bind(&DefaultWebClientWorker::SetAsDefault, this));
+  GetTaskRunner()->PostTask(
+      FROM_HERE, base::Bind(&DefaultWebClientWorker::SetAsDefault, this));
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -170,8 +170,36 @@ void DefaultWebClientWorker::OnCheckIsDefaultComplete(
 ///////////////////////////////////////////////////////////////////////////////
 // DefaultWebClientWorker, private:
 
+scoped_refptr<base::SequencedTaskRunner>
+DefaultWebClientWorker::GetTaskRunner() {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  static scoped_refptr<base::SequencedTaskRunner>* task_runner = nullptr;

[gab, 2017/05/30 19:22:21]

I think this can just be a

static scoped_refptr<base::SequencedTaskRunner> task_runner;

without being a scoped_refptr*

[Patrick Monette, 2017/05/31 16:13:57]

As per offline discussion, keeping this as a pointer since static variables must
be POD.

[robliao, 2017/07/14 21:13:13]

Wait, why must function static variables be POD? Are we just attempting to avoid
destruction here?

[Patrick Monette, 2017/07/14 21:25:34]

Prescribed by the style guide.
https://google.github.io/styleguide/cppguide.html#Static_and_Global_Variables

[robliao, 2017/07/14 21:30:57]

Got it. I thought that only applied at the global scope.

+
+  if (!task_runner) {
+#if defined(OS_WIN)
+    if (base::win::GetVersion() >= base::win::VERSION_WIN10) {
+      // TODO(pmonette): Windows 10's implementation uses a base::Timer which
+      // currently still requires a SingleThreadTaskRunner. Change this to a
+      // SequencedTaskRunner when crbug.com/552633 is fixed.
+      task_runner = new scoped_refptr<base::SequencedTaskRunner>(

[gab, 2017/05/30 19:22:21]

and then here you can forgo this new

[Patrick Monette, 2017/05/31 16:13:57]

Acknowledged.

+          base::CreateSingleThreadTaskRunnerWithTraits({base::MayBlock()}));
+    } else {
+      task_runner = new scoped_refptr<base::SequencedTaskRunner>(
+          base::CreateSequencedTaskRunnerWithTraits({base::MayBlock()}));
+    }
+#else   // defined(OS_WIN)
+    task_runner = new scoped_refptr<base::SequencedTaskRunner>(
+        base::CreateSequencedTaskRunnerWithTraits({base::MayBlock()}));
+#endif  // defined(OS_WIN)
+  }
+
+  return *task_runner;
+}
+
 void DefaultWebClientWorker::CheckIsDefault(bool is_following_set_as_default) {
-  DCHECK_CURRENTLY_ON(BrowserThread::FILE);
+  base::ThreadRestrictions::AssertIOAllowed();
+
   DefaultWebClientState state = CheckIsDefaultImpl();
   BrowserThread::PostTask(
       BrowserThread::UI, FROM_HERE,
@@ -180,7 +208,7 @@ void DefaultWebClientWorker::CheckIsDefault(bool is_following_set_as_default) {
 }
 
 void DefaultWebClientWorker::SetAsDefault() {
-  DCHECK_CURRENTLY_ON(BrowserThread::FILE);
+  base::ThreadRestrictions::AssertIOAllowed();
 
   // SetAsDefaultImpl will make sure the callback is executed exactly once.
   SetAsDefaultImpl(