Add virtual Initialize methods to PortAllocator and NetworkManager.

webrtc/p2p/base/portallocator.h

Index: webrtc/p2p/base/portallocator.h
diff --git a/webrtc/p2p/base/portallocator.h b/webrtc/p2p/base/portallocator.h
index 7d2a59f278ccb307ce0266ef89f65234eb291768..d03d29745c2d56f65dc8545c724aa93cca35db0d 100644
--- a/webrtc/p2p/base/portallocator.h
+++ b/webrtc/p2p/base/portallocator.h
@@ -221,8 +221,10 @@ class PortAllocatorSession : public sigslot::has_slots<> {
   friend class PortAllocator;
 };
 
-// Note that this class should only be used on one thread.
-// This includes calling the destructor.
+// This class can be constructed on any thread, but every other method
+// (including the destructor) should be called on a different thread.

[skvlad, 2016/06/23 22:15:55]

Do we have a way to verify that the "other thread" is always the same? like a
lazily initialized version of the ThreadChecker? 
It'd be good if we can enforce that requirement.

[Taylor Brandstetter, 2016/06/23 22:59:07]

Yeah, we can call "DetachFromThread" in the constructor so that it's attached
when the next method is called. This is the pattern Chrome uses. However...
Chrome currently still calls some methods from the wrong thread. I'll add the
thread checking once Chrome starts relying on Initialize, which will fix that.

+// This allows constructing a PortAllocator subclass on one thread and
+// passing it into an object that uses it on a different thread.
 class PortAllocator : public sigslot::has_slots<> {
  public:
   PortAllocator() :
@@ -232,10 +234,13 @@ class PortAllocator : public sigslot::has_slots<> {
       step_delay_(kDefaultStepDelay),
       allow_tcp_listen_(true),
       candidate_filter_(CF_ALL) {
-    // This will allow us to have old behavior on non webrtc clients.
   }
   virtual ~PortAllocator() {}
 
+  // This should be called on the PortAllocator's thread before the
+  // PortAllocator is used. Subclasses may override this if necessary.
+  virtual void Initialize() {}
+
   // Set STUN and TURN servers to be used in future sessions, and set
   // candidate pool size, as described in JSEP.
   //