Fix header size check in PseudoTcp::parse().

BUG=chromium:620694
TBR=pbos@webrtc.org

Review-Url: https://codereview.webrtc.org/2073963002
Cr-Commit-Position: refs/heads/master@{#13177}
(cherry picked from commit d1523ca38f98bd4e46faabcd2db57fad1e8ad154)

Review URL: https://codereview.webrtc.org/2083723002 .

Cr-Commit-Position: refs/branch-heads/52@{#6}
Cr-Branched-From: a376e70cf9d0df3c35d53533b454da542661775b-refs/heads/master@{#12798}
diff --git a/webrtc/p2p/base/pseudotcp.cc b/webrtc/p2p/base/pseudotcp.cc
index 42f6bad..4489374 100644
--- a/webrtc/p2p/base/pseudotcp.cc
+++ b/webrtc/p2p/base/pseudotcp.cc
@@ -573,7 +573,7 @@
 }
 
 bool PseudoTcp::parse(const uint8_t* buffer, uint32_t size) {
-  if (size < 12)
+  if (size < HEADER_SIZE)
     return false;
 
   Segment seg;