[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /ios/chrome/common/app_group.

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=olivierrobin@chromium.org

Bug: 874080
Change-Id: I74ae6a8f94ea54ee9738a67493dc545921dc99e9
Reviewed-on: https://chromium-review.googlesource.com/1191420
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594770}

ios/chrome/common/app_group/app_group_metrics_mainapp.mm

@@ -8,7 +8,7 @@
 
 #include "base/logging.h"
 #include "base/metrics/histogram_macros.h"
-#include "base/threading/thread_restrictions.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "ios/chrome/common/app_group/app_group_constants.h"
 #include "ios/chrome/common/app_group/app_group_metrics.h"
 
@@ -35,7 +35,7 @@ void RecordWidgetUsage() {
 }
 
 void ProcessPendingLogs(ProceduralBlockWithData callback) {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::WILL_BLOCK);
   NSFileManager* file_manager = [NSFileManager defaultManager];
   NSURL* store_url = [file_manager
       containerURLForSecurityApplicationGroupIdentifier:ApplicationGroup()];