Add functions to get/set rtp header extension by id.

webrtc/modules/rtp_rtcp/source/rtp_packet.h

Index: webrtc/modules/rtp_rtcp/source/rtp_packet.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_packet.h b/webrtc/modules/rtp_rtcp/source/rtp_packet.h
index 75c73cfbcc92ccb07f7a1331a6e6b3345f79d432..293e12b5ee8aeaef6a599b720c2c0d1c80635b7e 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_packet.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_packet.h
@@ -95,6 +95,19 @@ class Packet {
   template <typename Extension>
   bool ReserveExtension();
 
+  bool HasRawExtension(int id) const;

[nisse-webrtc, 2017/04/03 09:38:00]

|id| is the numerical value used on the wire (and associated with an uri via the
extensions map)? Doesn't seem entirely obvious from context, so I think it
deserves either a comment or a more verbose argument name.

In other places, in particular, RtpHeaderExtensionMap, we use the uint8_t for
the id. But RtpExtension uses an int. Can we get any consistency?

[danilchap, 2017/04/03 10:57:00]

Failed to find verbose enough argument name, so added a comment.
Moving towards |int|, specially for parameters.
uint8_t is not a good type for this value since it is not one-byte value on the
wire (it takes half-byte). Style recommend int for this kind of generic case
with run-time range checks.

+
+  // Returns place where extension with |id| is stored.
+  // Returns empty arrayview if extension is not present.

[nisse-webrtc, 2017/04/03 09:38:00]

The return convention makes sense, assuming that no valid extension can have
zero length. Is ArrayView used in this way elsewhere in the code?

[danilchap, 2017/04/03 10:57:00]

Yes, extension is not allowed to have 0 length.
Not right now: I try to use ArrayView more often though.
When return value can't be empty, then plan to use same convention, (i.e.
treating empty arrayview as a nullptr pointer.)

+  rtc::ArrayView<const uint8_t> GetRawExtension(int id) const;
+
+  // Allocates and store header extension. Returns true on success.
+  bool SetRawExtension(int id, rtc::ArrayView<const uint8_t> data);
+
+  // Allocates and returns place to store rtp header extension.
+  // Returns empty arrayview on failure.
+  rtc::ArrayView<uint8_t> AllocateRawExtension(int id, size_t length);
+
   // Reserve size_bytes for payload. Returns nullptr on failure.
   uint8_t* SetPayloadSize(size_t size_bytes);
   // Same as SetPayloadSize but doesn't guarantee to keep current payload.