Fix off-by-one error in DCHECK in ProducerFec.

webrtc/modules/rtp_rtcp/source/producer_fec_unittest.cc

Index: webrtc/modules/rtp_rtcp/source/producer_fec_unittest.cc
diff --git a/webrtc/modules/rtp_rtcp/source/producer_fec_unittest.cc b/webrtc/modules/rtp_rtcp/source/producer_fec_unittest.cc
index 8b84cf5712229c19df61b50dee55271f649f4961..f35058124bf63ad56b499132caa55643583a8b18 100644
--- a/webrtc/modules/rtp_rtcp/source/producer_fec_unittest.cc
+++ b/webrtc/modules/rtp_rtcp/source/producer_fec_unittest.cc
@@ -185,4 +185,22 @@ TEST_F(ProducerFecTest, BuildRedPacket) {
   delete packet;
 }
 
+TEST_F(ProducerFecTest, BuildRedPacketWithEmptyPayload) {
+  constexpr size_t kNumFrames = 1;
+  constexpr size_t kFrameId = 0;

[danilchap, 2016/09/01 07:59:17]

may be constexpr int kOffset = 0;
NextPacket name this parameter offset and use it as a random generator.
Doesn't feel related to Frame Id and definitely not a size_t.

Or, like in previous test do not introduce this constant since it is completely
irrelevant for this test.

[brandtr, 2016/09/01 09:29:18]

I'm not a fan of unnamed literals in the tests, but I still changed to that here
to avoid the confusion about what this offset means. The offset affects the
payload of the generated packets, and is typically used to separate the payloads
of different frames. Not really applicable here though.

About the type, I'd argue that it should be size_t in the signature of
NextPacket :)

+  constexpr size_t kPayloadLength = 0;
+
+  generator_.NewFrame(kNumFrames);
+  std::unique_ptr<test::RawRtpPacket> packet(
+      generator_.NextPacket(kFrameId, kPayloadLength));
+  std::unique_ptr<RedPacket> red_packet =
+      ProducerFec::BuildRedPacket(packet->data, packet->length - kRtpHeaderSize,
+                                  kRtpHeaderSize, kRedPayloadType);
+  EXPECT_EQ(packet->length + 1, red_packet->length());

[danilchap, 2016/09/01 07:59:17]

this + 1 is + kRedForFecHeaderLength?

[brandtr, 2016/09/01 09:29:18]

Yes, clarified.

+  VerifyHeader(packet->header.header.sequenceNumber,
+               packet->header.header.timestamp, kRedPayloadType,
+               packet->header.header.payloadType, red_packet.get(),
+               true);  // Marker bit set.
+}
+
 }  // namespace webrtc