Add read support of RtpStreamId/RepairedRtpStreamId header extensions.

webrtc/common_types.cc

Index: webrtc/common_types.cc
diff --git a/webrtc/common_types.cc b/webrtc/common_types.cc
index f5b487f6be0a5280c1e0a7c84f0037edd8e2db4f..cf92ae4a18959569b6b204bf9c4a8729964c6dc2 100644
--- a/webrtc/common_types.cc
+++ b/webrtc/common_types.cc
@@ -10,8 +10,9 @@
 
 #include "webrtc/common_types.h"
 
-#include <limits>
 #include <string.h>
+#include <limits>
+#include <type_traits>
 
 #include "webrtc/base/checks.h"
 #include "webrtc/base/stringutils.h"
@@ -20,6 +21,17 @@ namespace webrtc {
 
 StreamDataCounters::StreamDataCounters() : first_packet_time_ms(-1) {}
 
+constexpr size_t StreamId::kMaxSize;
+
+void StreamId::Set(const void* data, size_t size) {
+  RTC_DCHECK_LE(size, kMaxSize);

[nisse-webrtc, 2017/04/10 12:25:00]

I'd like an RTC_DCHECK(memchr(data, 0, size) == nullptr) here, to make sure
conversion makes sense. 

Or if you don't want to repeat this as a DCHECK here and a production check in
the parsing functions, you could let this method have a bool return value to
indicate success.

[danilchap, 2017/04/10 13:27:05]

Prefer to find those value acceptible. Added comment instead.

One of the reason - to make tests simpler and more natural: currently StreamId
created there from const char[N] where N would be string length including
terminating zero, but I still treat them as one-char shorter strings.

[nisse-webrtc, 2017/04/11 07:59:24]

Is that that the ArrayView constructor? I think that's a bit counter-intuitive
when it comes to string literals, I would have expected ArrayView<char>("foo")
to construct an array of length 3. 

If that can't be fixed, could tests go via a std::string instead, to get the
intended length?

[danilchap, 2017/04/11 10:05:26]

std::string_view would make more sense for StreamId, but that is available only
in c++17
When it is same complexity, I choose std::string in tests, but when testing
parsing RTPHeader, then comparing StreamId looks cleaner

+  memcpy(value_, data, size);
+  if (size < kMaxSize)
+    value_[size] = 0;
+}
+
+static_assert(std::is_trivially_destructible<StreamId>::value, "");
+
 RTPHeaderExtension::RTPHeaderExtension()
     : hasTransmissionTimeOffset(false),
       transmissionTimeOffset(0),