Add RtpPacket::IsExtensionReserved().

This is a small utility method to check whether an extension has been
reserved, so that can be checked before attempting to set an extension
without the need to actually try setting it and potentially failing
with warning loggins as a result.

Bug: webrtc:10633
Change-Id: Ie6f2c4f3f5e94a30dbf60aec6290ebee72681d9c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144461
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28455}

modules/rtp_rtcp/source/rtp_packet_unittest.cc

@@ -952,4 +952,25 @@ TEST(RtpPacketTest,
             kFeedbackRequest->sequence_count);
 }
 
+TEST(RtpPacketTest, IsExtensionReserved) {
+  // Register two extensions.
+  RtpPacketToSend::ExtensionManager extensions;
+  extensions.Register(kRtpExtensionTransmissionTimeOffset,
+                      kTransmissionOffsetExtensionId);
+  extensions.Register(kRtpExtensionAudioLevel, kAudioLevelExtensionId);
+
+  RtpPacketReceived packet(&extensions);
+
+  // Reserve slot for only one of them.
+  EXPECT_TRUE(packet.ReserveExtension<TransmissionOffset>());
+  // Non-registered extension cannot be reserved.
+  EXPECT_FALSE(packet.ReserveExtension<VideoContentTypeExtension>());
+
+  // Only the extension that is both registered and reserved matches
+  // IsExtensionReserved().
+  EXPECT_FALSE(packet.IsExtensionReserved<VideoContentTypeExtension>());
+  EXPECT_FALSE(packet.IsExtensionReserved<AudioLevel>());
+  EXPECT_TRUE(packet.IsExtensionReserved<TransmissionOffset>());
+}
+
 }  // namespace webrtc