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..6497e0cde5448149f331d209b4c10b7ccd6d4f3a 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,
-      base::Bind(&DefaultWebClientWorker::CheckIsDefault, this, false));
+  base::CreateSequencedTaskRunnerWithTraits({base::MayBlock()})

[gab, 2017/05/26 18:44:05]

Why does this need to be called on a sequence which isn't re-used? Does it use
SequencedTaskRunnerHandle or something?

If there are multiple check default calls, shouldn't they be sequenced between
themselves? And with SetAsDefault calls as well?

[Patrick Monette, 2017/05/29 18:54:14]

I decided to go with sequencing everything

+      ->PostTask(FROM_HERE, base::Bind(&DefaultWebClientWorker::CheckIsDefault,
+                                       this, false));
 }
 
 void DefaultWebClientWorker::StartSetAsDefault() {
-  BrowserThread::PostTask(
-      BrowserThread::FILE, FROM_HERE,
-      base::Bind(&DefaultWebClientWorker::SetAsDefault, this));
+  GetSetDefaultTaskRunner()->PostTask(
+      FROM_HERE, base::Bind(&DefaultWebClientWorker::SetAsDefault, this));
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -170,8 +170,38 @@ void DefaultWebClientWorker::OnCheckIsDefaultComplete(
 ///////////////////////////////////////////////////////////////////////////////
 // DefaultWebClientWorker, private:
 
+scoped_refptr<base::SequencedTaskRunner>
+DefaultWebClientWorker::GetSetDefaultTaskRunner() {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+#if defined(OS_WIN)
+  if (base::win::GetVersion() >= base::win::VERSION_WIN10) {
+    // On Windows 10, all the calls to SetAsDefault() must be done on the same
+    // SequencedTaskRunner (see OpenSystemSettingsHelper). Because the callers
+    // don't know each others, a global is shared between them.
+    //
+    // 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.

[gab, 2017/05/26 18:44:05]

Feels like we might as well just sequence all attempts to SetAsDefault() on all
platforms. Feels wrong to SetAsDefault() in parallel on any platform.

[Patrick Monette, 2017/05/29 18:54:14]

Sounds good.

+    static base::SingleThreadTaskRunner* task_runner = nullptr;

[gab, 2017/05/26 18:44:05]

Make this a scoped_refptr<> assign the result of base::Create...() directly into
it. Will avoid the whole AddRef() dance.

[Patrick Monette, 2017/05/29 18:54:14]

Done.

+
+    if (!task_runner) {
+      scoped_refptr<base::SingleThreadTaskRunner> new_task_runner =
+          base::CreateSingleThreadTaskRunnerWithTraits({base::MayBlock()});
+
+      new_task_runner->AddRef();
+      task_runner = new_task_runner.get();
+    }
+
+    return task_runner;
+  }
+#endif  // defined(OS_WIN)

[gab, 2017/05/26 18:44:05]

#else?

[Patrick Monette, 2017/05/29 18:54:14]

Done.

+  return base::CreateSequencedTaskRunnerWithTraits({base::MayBlock()});
+#endif  // !defined(OS_WIN)

[gab, 2017/05/26 18:44:05]

#endif  // defined(OS_WIN)

           ^ don't add '!', keep it the same as initial statement even in the
"else" step

[Patrick Monette, 2017/05/29 18:54:14]

Done.

+}
+
 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 +210,7 @@ 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(