Bug 1321878 P1 Don't exclude child process and IPC transit time from Performance API network timings. r=mayhemer

netwerk/protocol/http/HttpChannelChild.cpp

@@ -901,7 +901,14 @@ HttpChannelChild::OnStopRequest(const nsresult& channelStatus,
   mTransactionTimings.requestStart = timing.requestStart;
   mTransactionTimings.responseStart = timing.responseStart;
   mTransactionTimings.responseEnd = timing.responseEnd;
-  mAsyncOpenTime = timing.fetchStart;
+
+  // Do not overwrite or adjust the original mAsyncOpenTime.  We must use the
+  // original child process time in order to account for child side work and IPC
+  // transit overhead.  This depends on TimeStamp being equivalent across
+  // processes.  We work hard to ensure this on modern hardware, but there could
+  // be some variance on older devices.
+  MOZ_DIAGNOSTIC_ASSERT(mAsyncOpenTime <= timing.fetchStart);
+
   mRedirectStartTimeStamp = timing.redirectStart;
   mRedirectEndTimeStamp = timing.redirectEnd;
   mTransferSize = timing.transferSize;