Adds a flag to support "Virtual Time" to the blink scheduler

components/scheduler/common/scheduler_switches.cc

Index: components/scheduler/common/scheduler_switches.cc
diff --git a/components/scheduler/common/scheduler_switches.cc b/components/scheduler/common/scheduler_switches.cc
new file mode 100644
index 0000000000000000000000000000000000000000..f6d7cebc04f9bfcad8d7c5faf9d928f50c12bb76
--- /dev/null
+++ b/components/scheduler/common/scheduler_switches.cc
@@ -0,0 +1,16 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/scheduler/common/scheduler_switches.h"
+
+namespace scheduler {
+namespace switches {
+
+// Enable Virtualized time where the Render thread's time source skips forward
+// to the next scheduled delayed time if there is no more non-delayed work to be
+// done. This switch is intended only for use in headless mode.

[Sami, 2015/10/30 11:11:20]

There's nothing headless-mode specific here so perhaps we don't need to mention
it?

By the way, how do timers in background tabs work with virtualized time? It'll
be as if the throttling isn't there, right? Just thinking about crbug.com/485425
in the context of this.

Incidentally virtual time will nix the 1 ms setTimeout() clamping too.

[alex clarke (OOO till 29th), 2015/10/30 12:12:25]

Not entirely no.  Timer alignment will still happen but when DoWork runs out of
things to do virtual time will fast forward to the next timer, whatever that
happens to be.
Pretty sure this isn't what they want there.
Right.

[Sami, 2015/10/30 14:09:50]

Right, so essentially with virtual time there's no difference between a
foreground and a background tab (except how things interleave).
 
Why not? It will make timer based tests execute quickly regardless of tab
visibility. It could break timeout-based logic which may or may not be an issue.

+const char kEnableVirtualizedTime[] = "enable-virtualized-time";
+
+}  // namespace switches
+}  // namespace scheduler