Bug 1092978 - Part3: [Silk] add a PVsyncEvent sub-protocol in PBackground for cross process vsync event passing.
Bug 1092978 - Part4: [Silk] vsync-aligned refresh driver

1. add "gfx.vsync.refresh-driver" pref to turn on/off vsync-aligned refresh driver
2. observe vsync event to tick refresh driver.
[Silk] wip, PlatformVsyncTimer
add systrace tracer
add systrace tag for RD and vsync event
debug DidComposite() ipc message
debug CompositorParent composing time