[TaskScheduler]: Migrate off of ScopedAllowWait in /content/public/test

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It should have been replaced by :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=sky@chromium.org

Bug: 766678
Change-Id: I3b6193072a7049433d38a61ff62c1f34454c7b5e
Reviewed-on: https://chromium-review.googlesource.com/c/1324454
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606900}

content/public/test/browser_test_base.cc

@@ -154,7 +154,7 @@ BrowserTestBase::BrowserTestBase()
 BrowserTestBase::~BrowserTestBase() {
 #if defined(OS_ANDROID)
   // RemoteTestServer can cause wait on the UI thread.
-  base::ThreadRestrictions::ScopedAllowWait allow_wait;
+  base::ScopedAllowBaseSyncPrimitivesForTesting allow_wait;
   spawned_test_server_.reset();
 #endif