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

chrome/browser/shell_integration.h

 // 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.
 
 #ifndef CHROME_BROWSER_SHELL_INTEGRATION_H_
 #define CHROME_BROWSER_SHELL_INTEGRATION_H_
 
 #include <string>
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/strings/string16.h"
 #include "build/build_config.h"
 #include "ui/gfx/image/image_family.h"
 #include "url/gurl.h"
 
 namespace base {
 class CommandLine;
+class SequencedTaskRunner;
 }
 
 namespace shell_integration {
 
 // Sets Chrome as the default browser (only for the current user). Returns false
 // if this operation fails. This does not work on Windows version 8 or higher.
 // Prefer to use the DefaultBrowserWorker class below since it works on all OSs.
 bool SetAsDefaultBrowser();
 
 // Sets Chrome as the default client application for the given protocol
@@ skipping 91 matching lines @@
 base::string16 GetAppShortcutsSubdirName();
 #endif
 
 // The type of callback used to communicate processing state to consumers of
 // DefaultBrowserWorker and DefaultProtocolClientWorker.
 using DefaultWebClientWorkerCallback =
     base::Callback<void(DefaultWebClientState)>;
 
 //  Helper objects that handle checking if Chrome is the default browser
 //  or application for a url protocol on Windows and Linux, and also setting
-//  it as the default. These operations are performed asynchronously on the
-//  file thread since registry access (on Windows) or the preference database
-//  (on Linux) are involved and this can be slow.
+//  it as the default. These operations are performed asynchronously on a
+//  blocking sequence since registry access (on Windows) or the preference
+//  database (on Linux) are involved and this can be slow.
 //  By default, the worker will present the user with an interactive flow if
 //  required by the platform. This can be suppressed via
 //  set_interactive_permitted(), in which case an attempt to set Chrome as
 //  the default handler will silently fail on such platforms.
 class DefaultWebClientWorker
     : public base::RefCountedThreadSafe<DefaultWebClientWorker> {
  public:
   // Controls whether the worker can use user interaction to set the default
   // web client. If false, the set-as-default operation will fail on OS where
   // it is required.
@@ skipping 22 matching lines @@
   // |is_following_set_as_default| is true, |state| will be reported to UMA as
   // the result of the set-as-default operation.
   void OnCheckIsDefaultComplete(DefaultWebClientState state,
                                 bool is_following_set_as_default);
 
   // When false, the operation to set as default will fail for interactive
   // flows.
   bool interactive_permitted_ = true;
 
  private:
-  // Checks whether Chrome is the default web client. Always called on the
-  // FILE thread. When |is_following_set_as_default| is true, The default state
-  // will be reported to UMA as the result of the set-as-default operation.
+  scoped_refptr<base::SequencedTaskRunner> GetSetDefaultTaskRunner();

[gab, 2017/05/26 18:44:06]

"GetSet" sounds weird (took me a while to see this was "Get" "SetDefault")

maybe GetTaskRunnerForSetAsDefault()?

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

Good suggestion, but I used GetTaskRunner() since it is used for the
CheckIsDefault() operationt too.

+
+  // Checks whether Chrome is the default web client. Always called on the a

[gab, 2017/05/26 18:44:06]

"the a"

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

Done.

+  // blocking sequence. When |is_following_set_as_default| is true, The default
+  // state will be reported to UMA as the result of the set-as-default
+  // operation.
   void CheckIsDefault(bool is_following_set_as_default);
 
-  // Sets Chrome as the default web client. Always called on the FILE thread.
+  // Sets Chrome as the default web client. Always called on a blocking
+  // sequence.
   void SetAsDefault();
 
   // Implementation of CheckIsDefault() and SetAsDefault() for subclasses.
   virtual DefaultWebClientState CheckIsDefaultImpl() = 0;
 
   // The callback may be run synchronously or at an arbitrary time later on this
   // thread.
   // Note: Subclasses MUST make sure |on_finished_callback| is executed.
   virtual void SetAsDefaultImpl(const base::Closure& on_finished_callback) = 0;
 
@@ skipping 55 matching lines @@
   void SetAsDefaultImpl(const base::Closure& on_finished_callback) override;
 
   std::string protocol_;
 
   DISALLOW_COPY_AND_ASSIGN(DefaultProtocolClientWorker);
 };
 
 }  // namespace shell_integration
 
 #endif  // CHROME_BROWSER_SHELL_INTEGRATION_H_