Change rtc::TimeNanos and rtc::TimeMicros return value from uint64_t to int64_t.

Also updated types close to call sites.

BUG=webrtc:6733

Review-Url: https://codereview.webrtc.org/2514553003
Cr-Commit-Position: refs/heads/master@{#15255}
diff --git a/webrtc/base/opensslstreamadapter.cc b/webrtc/base/opensslstreamadapter.cc
index 5a7560e..6943cd3 100644
--- a/webrtc/base/opensslstreamadapter.cc
+++ b/webrtc/base/opensslstreamadapter.cc
@@ -64,7 +64,7 @@
 
 #ifdef OPENSSL_IS_BORINGSSL
 static void TimeCallback(const SSL* ssl, struct timeval* out_clock) {
-  uint64_t time = TimeNanos();
+  int64_t time = TimeNanos();
   out_clock->tv_sec = time / kNumNanosecsPerSec;
   out_clock->tv_usec = (time % kNumNanosecsPerSec) / kNumNanosecsPerMicrosec;
 }