[Documentation] RunAllTasksUntilIdle() is still useful in browser tests.

Deprecating was a bit too strong as this is still useful in few
browser tests (which don't have TestBrowserThreadBundle since it's a
unit test construct).

R=jam@chromium.org

Bug: 824431
Change-Id: Id1b44632aca4f02cbf579fe3fb2b0a91c2ac0ba4
Reviewed-on: https://chromium-review.googlesource.com/1054632Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557888}

content/public/test/test_utils.h

@@ -61,7 +61,8 @@ void RunAllPendingInMessageLoop();
 // rather than flushing entire threads.
 void RunAllPendingInMessageLoop(BrowserThread::ID thread_id);
 
-// Deprecated: Use TestBrowserThreadBundle::RunUntilIdle().
+// Runs all tasks on the current thread and TaskScheduler threads until idle.
+// Note: Prefer TestBrowserThreadBundle::RunUntilIdle() in unit tests.
 void RunAllTasksUntilIdle();
 
 // Get task to quit the given RunLoop. It allows a few generations of pending