Rename rtc::Time64 --> rtc::TimeMillis.

In the discussion on https://codereview.webrtc.org/1888593004/, a more
decriptive name was suggested for Time64.

BUG=webrtc:5740

Review-Url: https://codereview.webrtc.org/1923213002
Cr-Commit-Position: refs/heads/master@{#12594}
diff --git a/webrtc/p2p/base/stunrequest.cc b/webrtc/p2p/base/stunrequest.cc
index b12e7cb..b75bcf6 100644
--- a/webrtc/p2p/base/stunrequest.cc
+++ b/webrtc/p2p/base/stunrequest.cc
@@ -194,7 +194,7 @@
 }
 
 int StunRequest::Elapsed() const {
-  return static_cast<int>(rtc::Time64() - tstamp_);
+  return static_cast<int>(rtc::TimeMillis() - tstamp_);
 }
 
 
@@ -213,7 +213,7 @@
     return;
   }
 
-  tstamp_ = rtc::Time64();
+  tstamp_ = rtc::TimeMillis();
 
   rtc::ByteBufferWriter buf;
   msg_->Write(&buf);