scheduler: Turn expensive task blocking into a finch trial

components/scheduler/renderer/renderer_scheduler.cc

Index: components/scheduler/renderer/renderer_scheduler.cc
diff --git a/components/scheduler/renderer/renderer_scheduler.cc b/components/scheduler/renderer/renderer_scheduler.cc
index aedc3c6f40bf0c7ba8c43fcf5b13f304a6980503..e39260ead6efd50efa45a0306291f5cf754b0c1c 100644
--- a/components/scheduler/renderer/renderer_scheduler.cc
+++ b/components/scheduler/renderer/renderer_scheduler.cc
@@ -5,6 +5,7 @@
 #include "components/scheduler/renderer/renderer_scheduler.h"
 
 #include "base/command_line.h"
+#include "base/feature_list.h"
 #include "base/message_loop/message_loop.h"
 #include "base/time/default_tick_clock.h"
 #include "base/trace_event/trace_event.h"
@@ -15,6 +16,10 @@
 #include "components/scheduler/renderer/renderer_scheduler_impl.h"
 
 namespace scheduler {
+namespace {
+const base::Feature kExpensiveTaskBlockingPolicyFeature{
+    "SchedulerExpensiveTaskBlocking", base::FEATURE_DISABLED_BY_DEFAULT};
+}
 
 RendererScheduler::RendererScheduler() {
 }
@@ -33,16 +38,20 @@ scoped_ptr<RendererScheduler> RendererScheduler::Create() {
   base::trace_event::TraceLog::GetCategoryGroupEnabled(
       TRACE_DISABLED_BY_DEFAULT("renderer.scheduler.debug"));
 
+  scoped_ptr<RendererSchedulerImpl> scheduler;
   base::MessageLoop* message_loop = base::MessageLoop::current();
   base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
   if (command_line->HasSwitch(switches::kEnableVirtualizedTime)) {
-    return make_scoped_ptr(new RendererSchedulerImpl(
+    scheduler.reset(new RendererSchedulerImpl(
         VirtualTimeTqmDelegate::Create(message_loop, base::TimeTicks::Now())));
   } else {
-    return make_scoped_ptr(
-        new RendererSchedulerImpl(SchedulerTqmDelegateImpl::Create(
-            message_loop, make_scoped_ptr(new base::DefaultTickClock()))));
+    scheduler.reset(new RendererSchedulerImpl(SchedulerTqmDelegateImpl::Create(
+        message_loop, make_scoped_ptr(new base::DefaultTickClock()))));
   }
+
+  scheduler->SetExpensiveTaskBlockingAllowed(

[alex clarke (OOO till 29th), 2016/01/13 17:18:30]

Doesn't have to be this patch but perhaps we might want a command line flag to
disable this too. WDYT?

[Sami, 2016/01/13 17:25:46]

Conveniently this already hooks up to
--enable-features=... and --disable-features=... so things Just Work.

+      base::FeatureList::IsEnabled(kExpensiveTaskBlockingPolicyFeature));
+  return make_scoped_ptr<RendererScheduler>(scheduler.release());
 }
 
 // static