Fix crash on suspend for chrome os

If RendererFreezer::SuspendImminent was called multiple times before it
had a chance to report readiness, it would post multiple tasks to do so
on the current message loop.  However the callback to report readiness
would be cleared after the first task ran and would cause a crash when
the next task tried to run it.  Fix this by not posting new tasks while
a callback is pending and just update the callback instead.

BUG=414396
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>

Review URL: https://codereview.chromium.org/550093004

Cr-Commit-Position: refs/heads/master@{#294916}

chrome/browser/chromeos/power/renderer_freezer.cc

@@ -23,14 +23,21 @@ const char kThawCommand[] = "THAWED";
 }  // namespace
 
 void RendererFreezer::SuspendImminent() {
-  suspend_readiness_callback_ = DBusThreadManager::Get()
-                                    ->GetPowerManagerClient()
-                                    ->GetSuspendReadinessCallback();
-
+  // SuspendImminent() might end up being called multiple times before we run
+  // OnReadyToSuspend() (crbug.com/414396).  In case a callback is already
+  // pending, we only store the new callback and do nothing else.
+  if (suspend_readiness_callback_.is_null()) {
+    // There is no callback pending so post the task.
     base::MessageLoop::current()->PostTask(
         FROM_HERE,
         base::Bind(&RendererFreezer::OnReadyToSuspend,
                    weak_factory_.GetWeakPtr()));
+  }
+
+  // Always update the callback because only the most recent one matters.
+  suspend_readiness_callback_ = DBusThreadManager::Get()
+                                    ->GetPowerManagerClient()
+                                    ->GetSuspendReadinessCallback();
 }
 
 void RendererFreezer::SuspendDone(const base::TimeDelta& sleep_duration) {
@@ -56,7 +63,7 @@ void RendererFreezer::OnReadyToSuspend() {
     frozen_ = true;
   }
 
-  DCHECK(!suspend_readiness_callback_.is_null());
+  CHECK(!suspend_readiness_callback_.is_null());  // crbug.com/414396
   suspend_readiness_callback_.Run();
   suspend_readiness_callback_.Reset();
 }