Always return 0 from QuicPickServerPortForTestsOrDieImpl

to force the OS to pick an unused port and hence avoid flakiness
in QUIC EndToEndTests.

Bug: 997325
Change-Id: I1c6eecac8e599863ab60077f1692d209fe055dc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1776603
Commit-Queue: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Auto-Submit: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691754}

net/quic/platform/impl/quic_port_utils_impl.cc

@@ -9,7 +9,7 @@
 namespace quic {
 
 int QuicPickServerPortForTestsOrDieImpl() {
-  return 12345 + (QuicRandom::GetInstance()->RandUint64() % 20000);
+  return 0;  // Let the OS find an unused port.
 }
 
 void QuicRecyclePortImpl(int port) {}