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/base/timeutils_unittest.cc b/webrtc/base/timeutils_unittest.cc
index 7e342d0..636c408 100644
--- a/webrtc/base/timeutils_unittest.cc
+++ b/webrtc/base/timeutils_unittest.cc
@@ -144,6 +144,14 @@
   EXPECT_TRUE(0 <= microseconds && microseconds < 1000000);
 }
 
+TEST(TimeTest, TestTimeDiff64) {
+  int64_t ts_diff = 100;
+  int64_t ts_earlier = rtc::Time64();
+  int64_t ts_later = ts_earlier + ts_diff;
+  EXPECT_EQ(ts_diff, rtc::TimeDiff(ts_later, ts_earlier));
+  EXPECT_EQ(-ts_diff, rtc::TimeDiff(ts_earlier, ts_later));
+}
+
 class TimestampWrapAroundHandlerTest : public testing::Test {
  public:
   TimestampWrapAroundHandlerTest() {}