[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /chrome/browser/apps/app_shim/app_shim_host_manager_mac.mm.

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=thakis@chromium.org

Bug: 874080
Change-Id: I449d164d700298ec166451458a5f9b135514156d
Reviewed-on: https://chromium-review.googlesource.com/1190933Reviewed-by: ccameron <ccameron@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595857}

chrome/browser/apps/app_shim/app_shim_host_manager_mac.mm

@@ -12,6 +12,7 @@
 #include "base/logging.h"
 #include "base/path_service.h"
 #include "base/task/post_task.h"
+#include "base/threading/scoped_blocking_call.h"
 #include "chrome/browser/apps/app_shim/app_shim_handler_mac.h"
 #include "chrome/browser/apps/app_shim/app_shim_host_mac.h"
 #include "chrome/browser/apps/app_shim/extension_app_shim_handler_mac.h"
@@ -40,7 +41,7 @@ base::FilePath GetDirectoryInTmpTemplate(const base::FilePath& user_data_dir) {
 void DeleteSocketFiles(const base::FilePath& directory_in_tmp,
                        const base::FilePath& symlink_path,
                        const base::FilePath& version_path) {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
 
   // Delete in reverse order of creation.
   if (!version_path.empty())
@@ -95,7 +96,7 @@ AppShimHostManager::~AppShimHostManager() {
 }
 
 void AppShimHostManager::InitOnBackgroundThread() {
-  base::AssertBlockingAllowed();
+  base::ScopedBlockingCall scoped_blocking_call(base::BlockingType::MAY_BLOCK);
   base::FilePath user_data_dir;
   if (!base::PathService::Get(chrome::DIR_USER_DATA, &user_data_dir))
     return;