[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /chrome/browser/safe_browsing/chrome_cleaner.

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=csharp@chromium.org

Bug: 874080
Change-Id: I131400a7be516f5ea30857e04528550ea5472fae
Reviewed-on: https://chromium-review.googlesource.com/1191230Reviewed-by: Chris Sharp <csharp@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588922}

chrome/browser/safe_browsing/chrome_cleaner/chrome_cleaner_controller_impl_win.cc

@@ -20,7 +20,6 @@
 #include "base/metrics/histogram_macros.h"
 #include "base/task/post_task.h"
 #include "base/task/task_traits.h"
-#include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/component_updater/sw_reporter_installer_win.h"
@@ -89,8 +88,6 @@ enum IPCDisconnectedHistogramValue {
 base::FilePath VerifyAndRenameDownloadedCleaner(
     base::FilePath downloaded_path,
     ChromeCleanerFetchStatus fetch_status) {
-  base::AssertBlockingAllowed();
-
   if (downloaded_path.empty() || !base::PathExists(downloaded_path))
     return base::FilePath();