Remove a DCHECK from rtt_stats.h.

Merege internal change 195881570

R=rch@chromium.org

Bug: 840734
Change-Id: I2002111d42876e8c2b0d66850be67dea3c67cfdb
Reviewed-on: https://chromium-review.googlesource.com/1051728Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557222}

net/quic/core/congestion_control/rtt_stats.h

@@ -81,7 +81,6 @@ class QUIC_EXPORT_PRIVATE RttStats {
   }
 
   void set_initial_max_ack_delay(QuicTime::Delta initial_max_ack_delay) {
-    DCHECK(max_ack_delay_.IsZero());
     max_ack_delay_ = std::max(max_ack_delay_, initial_max_ack_delay);
   }