[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /ios/chrome/browser/ui/image_util.

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=gambard@chromium.org

Bug: 874080
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I14f3200988ea3482fbccd94e0ff43e46dd192504
Reviewed-on: https://chromium-review.googlesource.com/1191130Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586715}

ios/chrome/browser/ui/image_util/image_saver.mm

@@ -11,7 +11,7 @@
 #include "base/format_macros.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/task/post_task.h"
-#include "base/threading/thread_restrictions.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "components/image_fetcher/ios/ios_image_data_fetcher_wrapper.h"
 #include "components/strings/grit/components_strings.h"
 #import "ios/chrome/browser/ui/alert_coordinator/alert_coordinator.h"
@@ -123,7 +123,8 @@
       {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
        base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN},
       base::BindOnce(^{
-        base::AssertBlockingAllowed();
+        base::ScopedBlockingCall scoped_blocking_call(
+            base::BlockingType::MAY_BLOCK);
 
         NSString* fileName = [[[NSProcessInfo processInfo] globallyUniqueString]
             stringByAppendingString:fileExtension];
@@ -148,7 +149,8 @@
                   {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
                    base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN},
                   base::BindOnce(^{
-                    base::AssertBlockingAllowed();
+                    base::ScopedBlockingCall scoped_blocking_call(
+                        base::BlockingType::MAY_BLOCK);
                     if (completion)
                       completion(success, error);