Reland "[MessageLoop] Lock-free ScheduleWork() scheme"

This is a reland of f7da13a2
(original change in PS1)

The original change was missing acquire ordering in
DisconnectFromParent(). This is necessary in order for
the disconnecting thread to see all memory side-effects
previously made by other threads (or some side-effects
of message_loop_->ScheduleWork() could racily come in
after ~MessageLoop()).

Also removed the DCHECK that
|operations_state_ == kDisconnectedBit| at the end of
DisconnectFromParent() as it was incorrect. A racy
BeforeOperation() call can make it 1->3 (and no-op) after
DisconnectFromParent() made it 0->1.

And lastly, added a scoped allowance to always allow the
very fast wait instead of requiring callers to know about
this implementation detail of MessageLoop (and reverted
changes to //net).

Original change's description:
> [MessageLoop] Lock-free ScheduleWork() scheme
>
> The Lock is causing hangs because of priority inversion
> mixed with priority boosting (ScheduleWork() tends to
> boost the destination thread which may deschedule the
> posting thread; if the posting thread is a background
> thread this boost-induded-desched-while-holding-lock
> can cause a livelock). See https://crbug.com/890978#c10
> for example crashes catching this.
>
> The Lock was only necessary for startup/shutdown and is
> being replaced by a lock-free atomic scheme in this CL.
>
> MessagePump::ScheduleWork() itself was already thread-safe
> (but the Android impl did unnecessarily check a non-atomic bool)
>
> This adds a WaitableEvent in ~MessageLoop(); hence the requirement
> for a wait-allowance in net's EmbeddedTestServer.
>
> TBR=zhongyi@chromium.org (embedded_test_server.cc side-effects)
>
> Bug: 890978, 874237
> Change-Id: I0916e5a99035a935b0a23a770af256f334e78c43
> Reviewed-on: https://chromium-review.googlesource.com/c/1278631
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: François Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#601600}

Bug: 890978, 874237, 897925
Change-Id: I17c515f9a3169bbdfc303a4b259f34097e31730d
Reviewed-on: https://chromium-review.googlesource.com/c/1297129Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602166}

base/message_loop/message_loop.h

@@ -260,8 +260,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate,
   // destructor to make sure all the task's destructors get called.
   void DeletePendingTasks();
 
-  // Wakes up the message pump. Can be called on any thread. The caller is
-  // responsible for synchronizing ScheduleWork() calls.
+  // Wakes up the message pump. Thread-safe (and callers should avoid holding a
+  // Lock at all cost while making this call as some platforms' priority
+  // boosting features have been observed to cause the caller to get descheduled
+  // : https://crbug.com/890978).
   void ScheduleWork();
 
   // Returns |next_run_time| capped at 1 day from |recent_time_|. This is used

base/message_loop/message_pump.h

@@ -112,9 +112,11 @@ class BASE_EXPORT MessagePump {
   virtual void Quit() = 0;
 
   // Schedule a DoWork callback to happen reasonably soon.  Does nothing if a
-  // DoWork callback is already scheduled.  This method may be called from any
-  // thread.  Once this call is made, DoWork should not be "starved" at least
-  // until it returns a value of false.
+  // DoWork callback is already scheduled. Once this call is made, DoWork should
+  // not be "starved" at least until it returns a value of false. Thread-safe
+  // (and callers should avoid holding a Lock at all cost while making this call
+  // as some platforms' priority boosting features have been observed to cause
+  // the caller to get descheduled : https://crbug.com/890978).
   virtual void ScheduleWork() = 0;
 
   // Schedule a DoDelayedWork callback to happen at the specified time,

base/message_loop/message_pump_android.cc

@@ -278,9 +278,6 @@ void MessagePumpForUI::Quit() {
 }
 
 void MessagePumpForUI::ScheduleWork() {
-  if (ShouldQuit())
-    return;
-
   // Write (add) 1 to the eventfd. This tells the Looper to wake up and call our
   // callback, allowing us to run tasks. This also allows us to detect, when we
   // clear the fd, whether additional work was scheduled after we finished

base/threading/thread_restrictions.h

@@ -149,6 +149,7 @@ class TaskTracker;
 
 class AdjustOOMScoreHelper;
 class GetAppOutputScopedAllowBaseSyncPrimitives;
+class MessageLoop;
 class SimpleThread;
 class StackSamplingProfiler;
 class Thread;
@@ -353,6 +354,7 @@ class BASE_EXPORT ScopedAllowBaseSyncPrimitivesOutsideBlockingScope {
       ThreadRestrictionsTest,
       ScopedAllowBaseSyncPrimitivesOutsideBlockingScopeResetsState);
   friend class ::KeyStorageLinux;
+  friend class base::MessageLoop;
   friend class content::SynchronousCompositor;
   friend class content::SynchronousCompositorHost;
   friend class content::SynchronousCompositorSyncCallBridge;