[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /content/browser/devtools.

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=caseq@chromium.org

Bug: 874080
Change-Id: Ib5b94340df750c17a0a3ab0b8dd76f4fc70c5c5e
Reviewed-on: https://chromium-review.googlesource.com/1191678Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592254}

content/browser/devtools/devtools_stream_file.cc

@@ -12,7 +12,6 @@
 #include "base/task/lazy_task_runner.h"
 #include "base/task/post_task.h"
 #include "base/third_party/icu/icu_utf.h"
-#include "base/threading/thread_restrictions.h"
 #include "content/public/browser/browser_thread.h"
 #include "storage/browser/fileapi/file_system_context.h"
 
@@ -46,7 +45,6 @@ DevToolsStreamFile::~DevToolsStreamFile() {
 
 bool DevToolsStreamFile::InitOnFileSequenceIfNeeded() {
   DCHECK(task_runner_->RunsTasksInCurrentSequence());
-  base::AssertBlockingAllowed();
   if (had_errors_)
     return false;
   if (file_.IsValid())

content/browser/devtools/protocol/devtools_download_manager_delegate.cc

@@ -7,7 +7,6 @@
 #include "base/bind.h"
 #include "base/files/file_util.h"
 #include "base/task/post_task.h"
-#include "base/threading/thread_restrictions.h"
 #include "content/browser/devtools/protocol/devtools_download_manager_helper.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/browser_thread.h"
@@ -159,7 +158,6 @@ void DevToolsDownloadManagerDelegate::GenerateFilename(
     const std::string& mime_type,
     const base::FilePath& suggested_directory,
     const FilenameDeterminedCallback& callback) {
-  base::AssertBlockingAllowed();
   base::FilePath generated_name =
       net::GenerateFileName(url, content_disposition, std::string(),
                             suggested_filename, mime_type, "download");