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/base/timeutils.cc b/webrtc/base/timeutils.cc
index de3e6af..b222470 100644
--- a/webrtc/base/timeutils.cc
+++ b/webrtc/base/timeutils.cc
@@ -82,7 +82,7 @@
   return static_cast<uint32_t>(TimeNanos() / kNumNanosecsPerMillisec);
 }
 
-int64_t Time64() {
+int64_t TimeMillis() {
   return static_cast<int64_t>(TimeNanos() / kNumNanosecsPerMillisec);
 }