Using 64-bit timestamp to replace the 32-bit one in webrtc/p2p.
Also changed from unsigned to signed integer per the style guide.
By the way, I kept all delta-times to be 32-bit int.

The only things left in the p2p dir are
1. proberprober/main.cc where Time() is used as the input for a random number.
2. pseudotcp.cc: where 32-bit time info is sent over the wire.

BUG=webrtc:5636

Review URL: https://codereview.webrtc.org/1793553002

Cr-Commit-Position: refs/heads/master@{#12019}
diff --git a/webrtc/p2p/base/stunrequest.h b/webrtc/p2p/base/stunrequest.h
index 4217a81..f6d216d 100644
--- a/webrtc/p2p/base/stunrequest.h
+++ b/webrtc/p2p/base/stunrequest.h
@@ -101,7 +101,7 @@
   const StunMessage* msg() const;
 
   // Time elapsed since last send (in ms)
-  uint32_t Elapsed() const;
+  int Elapsed() const;
 
  protected:
   int count_;
@@ -129,7 +129,7 @@
 
   StunRequestManager* manager_;
   StunMessage* msg_;
-  uint32_t tstamp_;
+  int64_t tstamp_;
 
   friend class StunRequestManager;
 };