[TaskEnvironment] Disable SequenceManager anti-starvation logic

This is the fix to the issue highlighted at
https://chromium-review.googlesource.com/c/chromium/src/+/1938067

Anti-starvation on the UI/IO threads is highly problematic
per unexpected reorder of base::TaskPriority. BrowserTaskExecutors
already disables this. TaskEnvironment needs to do the same.

R=alexclarke@chromium.org

Bug: 1013535
Change-Id: Ief82ff8a80813455e60bb6ed53fdb23228f68f7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948093
Auto-Submit: Gabriel Charette <gab@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721400}

base/test/task_environment.cc

@@ -74,6 +74,7 @@ CreateSequenceManagerForMainThreadType(
       MessagePump::Create(type),
       base::sequence_manager::SequenceManager::Settings::Builder()
           .SetMessagePumpType(type)
+          .SetAntiStarvationLogicForPrioritiesDisabled(true)
           .Build());
 }