Postpone native window initialization and display.

Runtime::AttachWindow() is time costing in Browser::UI thread.
Unfortunately during this peroid, extension process requires sync talk
with Browser::UI to set up dbus proxy, moreover, render process
initialization is also delayed due to LaunchInternal() can't be called
on time.

The PR leaves time slot for those operations so that EP and RP can be
initialized timely.

From local test, it can shorten warm start time by ~65ms.

BUG=XWALK-2520