[TaskScheduler]: Migrate off of ScopedAllowWait in /base/task/task_scheduler

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

It should have been replaced by :
 * base::ScopedAllowBaseSyncPrimitivesForTesting in test files.
 * base::ScopedAllowSyncPrimitives in non-test files
 * base::ScopedAllowSyncPrimitivesOutsideBlockingScope 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=fdoray@chromium.org

Bug: 766678
Change-Id: I49ce1451e0a57c832a2ef09d00364aba4b6a69e9
Reviewed-on: https://chromium-review.googlesource.com/c/1324397Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607253}

base/task/task_scheduler/task_tracker.cc

@@ -691,7 +691,7 @@ void TaskTracker::PerformShutdown() {
   // It is safe to access |shutdown_event_| without holding |lock_| because the
   // pointer never changes after being set above.
   {
-    base::ThreadRestrictions::ScopedAllowWait allow_wait;
+    base::ScopedAllowBaseSyncPrimitives allow_wait;
     shutdown_event_->Wait();
   }
 }