Increase WebRTC socket send buffer to 256k.

WebRTC has a throttling mechanism to ensure we don't push too much data into the system socket. The buffer size doesn't seem to be set correctly and it was experienced video freeze from time to time as the throttling mechanism kicks in too frequently and causes video packets dropped, even though system still has more capacity to handle it.

BUG=427555

Review URL: https://codereview.chromium.org/678103003

Cr-Commit-Position: refs/heads/master@{#301580}

content/renderer/p2p/ipc_socket_factory.cc

@@ -62,7 +62,8 @@ bool JingleSocketOptionToP2PSocketOption(rtc::Socket::Option option,
 }
 
 // TODO(miu): This needs tuning.  http://crbug.com/237960
-const size_t kMaximumInFlightBytes = 64 * 1024;  // 64 KB
+// http://crbug.com/427555
+const size_t kMaximumInFlightBytes = 256 * 1024;  // 256 KB
 
 // IpcPacketSocket implements rtc::AsyncPacketSocket interface
 // using P2PSocketClient that works over IPC-channel. It must be used