[TaskScheduler]: Migrate off of ScopedAllowWait in /content/browser/browser_main_loop.cc

base::ThreadRestrictions::ScopedAllowWait is deprecated in favor of its more
explicit counterpart.

It should have been replaced by :
 * base::ScopedAllowBaseSyncPrimitivesForTesting in test files.
 * base::ScopedAllowBaseSyncPrimitives in non-test files
 * base::ScopedAllowBaseSyncPrimitivesOutsideBlockingScope when it's used on threads
   that don't allow blocking
The last one is strongly frowned upon but this CL aims to document existing
behavior rather than address it. Owners are encouraged to follow-up by fixing
unnecessary waits and more particularly unnecessary waits
outside-blocking-scope.

Note: The non-for-testing versions require friend'ing in thread_restrictions.h
but care was taken to add these friends ahead of git cl split (since it wasn't
possible to do a line-by-line associated CL split).
Refer to the top-level CL if necessary :
https://chromium-review.googlesource.com/c/chromium/src/+/1288533

Please CQ if LGTY!

This CL was uploaded by git cl split.

R=gab@chromium.org

Bug: 766678
Change-Id: I51b024db95473edf6ee3efb82879c95be92b9191
Reviewed-on: https://chromium-review.googlesource.com/c/1325219Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606490}

content/browser/browser_main_loop.cc

@@ -1088,7 +1088,7 @@ void BrowserMainLoop::ShutdownThreadsAndCleanUp() {
     save_file_manager_->Shutdown();
 
   {
-    base::ThreadRestrictions::ScopedAllowWait allow_wait_for_join;
+    base::ScopedAllowBaseSyncPrimitives allow_wait_for_join;
     {
       TRACE_EVENT0("shutdown", "BrowserMainLoop::Subsystem:IOThread");
       ResetThread_IO(std::move(io_thread_));