Move kMaxSegmentSize to constant file (net/)

When building using Jumbo unnamed namespaces gets merged
and variables with the same name conflict. This happens
for the variables kMaxSegmentsSize in:
net/third_party/quic/core/congestion_control/bbr_sender.cc
net/third_party/quic/core/congestion_control/prr_sender.cc

This commit solves the issue by moving the constant to
net/third_party/quic/core/quic_constants.h.

Bug: 772146
Change-Id: I07da21c2bc8c96f6ed5cc83e2724ae948d80a9be
Reviewed-on: https://chromium-review.googlesource.com/1119689
Commit-Queue: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571814}
diff --git a/net/third_party/quic/core/congestion_control/bbr_sender.cc b/net/third_party/quic/core/congestion_control/bbr_sender.cc
index e02fcf3..094f683 100644
--- a/net/third_party/quic/core/congestion_control/bbr_sender.cc
+++ b/net/third_party/quic/core/congestion_control/bbr_sender.cc
@@ -20,7 +20,6 @@
 
 namespace {
 // Constants based on TCP defaults.
-const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
 // The minimum CWND to ensure delayed acks don't reduce bandwidth measurements.
 // Does not inflate the pacing rate.
 const QuicByteCount kDefaultMinimumCongestionWindow = 4 * kMaxSegmentSize;
diff --git a/net/third_party/quic/core/congestion_control/prr_sender.cc b/net/third_party/quic/core/congestion_control/prr_sender.cc
index a39cfd7..e4e256a 100644
--- a/net/third_party/quic/core/congestion_control/prr_sender.cc
+++ b/net/third_party/quic/core/congestion_control/prr_sender.cc
@@ -8,11 +8,6 @@
 
 namespace quic {
 
-namespace {
-// Constant based on TCP defaults.
-const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
-}  // namespace
-
 PrrSender::PrrSender()
     : bytes_sent_since_loss_(0),
       bytes_delivered_since_loss_(0),
diff --git a/net/third_party/quic/core/quic_constants.h b/net/third_party/quic/core/quic_constants.h
index e13e5da6..73e83d1 100644
--- a/net/third_party/quic/core/quic_constants.h
+++ b/net/third_party/quic/core/quic_constants.h
@@ -37,6 +37,7 @@
 // Default maximum packet size used in the Linux TCP implementation.
 // Used in QUIC for congestion window computations in bytes.
 const QuicByteCount kDefaultTCPMSS = 1460;
+const QuicByteCount kMaxSegmentSize = kDefaultTCPMSS;
 
 // We match SPDY's use of 32 (since we'd compete with SPDY).
 const QuicPacketCount kInitialCongestionWindow = 32;