[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /chrome/browser/background/background_mode_manager_mac.mm.

This CL was created by replacing calls to AssertBlockingAllowed()
with instantiations of ScopedBlockingCall(MAY_BLOCK).
I kindly ask the reviewer to make sure of the following:
  - ScopedBlockingCall is instantiated in a scope with minimal CPU usage.
    If this is not the case, ScopedBlockingCall should be instantiated
    closer to the blocking call. See scoped_blocking_call.h for more
    info. Please let me know when/where the blocking call happens if this needs
    to be changed.
  - Parameter |blocking_type| matches expectation (MAY_BLOCK/WILL_BLOCK). See
    BlockingType for more info. While I assumed MAY_BLOCK by default, that might
    not be the best fit if we know that this callsite is guaranteed to block.
  - The ScopedBlockingCall's scope covers the entirety of the blocking operation
    previously asserted against by the AssertBlockingAllowed().

This CL was uploaded by git cl split.

R=avi@chromium.org

Bug: 874080
Change-Id: Ic807c6eb4e22dc7ebdd34e2240e1fe6484e44174
Reviewed-on: https://chromium-review.googlesource.com/1191232Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593676}

chrome/browser/background/background_mode_manager_mac.mm

@@ -7,7 +7,7 @@
 #include "base/mac/mac_util.h"
 #include "base/sequenced_task_runner.h"
 #include "base/task/post_task.h"
-#include "base/threading/thread_restrictions.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "chrome/browser/background/background_mode_manager.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/common/chrome_switches.h"
@@ -32,7 +32,7 @@ void SetCreatedLoginItemPrefOnUIThread() {
 }
 
 void DisableLaunchOnStartupOnWorkerThread() {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
   // If the LoginItem is not hidden, it means it's user created, so don't
   // delete it.
   bool is_hidden = false;
@@ -41,7 +41,7 @@ void DisableLaunchOnStartupOnWorkerThread() {
 }
 
 void CheckForUserRemovedLoginItemOnWorkerThread() {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
   if (!base::mac::CheckLoginItemStatus(NULL)) {
     // There's no LoginItem, so set the kUserRemovedLoginItem pref.
     base::PostTaskWithTraits(FROM_HERE, {BrowserThread::UI},
@@ -50,7 +50,7 @@ void CheckForUserRemovedLoginItemOnWorkerThread() {
 }
 
 void EnableLaunchOnStartupOnWorkerThread(bool need_migration) {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
   if (need_migration) {
     // This is the first time running Chrome since the kChromeCreatedLoginItem
     // pref was added. Initialize the status of this pref based on whether