[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /device/bluetooth/bluez.

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=rkc@chromium.org

Bug: 874080
Change-Id: I4ab9d5ae6b94c448f3e30e6e9c607fb88a1078bf
Reviewed-on: https://chromium-review.googlesource.com/1191183Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595086}

device/bluetooth/bluez/bluetooth_socket_bluez.cc

@@ -21,7 +21,7 @@
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_util.h"
 #include "base/task_runner_util.h"
-#include "base/threading/thread_restrictions.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "dbus/bus.h"
 #include "dbus/object_path.h"
@@ -450,7 +450,7 @@ void BluetoothSocketBlueZ::DoNewConnection(
     const bluez::BluetoothProfileServiceProvider::Delegate::Options& options,
     const ConfirmationCallback& callback) {
   DCHECK(socket_thread()->task_runner()->RunsTasksInCurrentSequence());
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
 
   if (!fd.is_valid()) {
     LOG(WARNING) << uuid_.canonical_value() << " :" << fd.get()