Fix circular dependencies in webrtc_common.

One reason for the circular deps is that common_types.h is a
historical dumping ground for various structs and defines that
are believed to be generally useful. I tried moving things out
that did not appear to be used downstream (StreamCounters,
RtpCounters etc) and moved the things that seemed used
(RtpHeader + supporting structs) to a new file api/rtp_headers.h.
This makes their place in the api more clear while moving out
the things that don't belong in the API in the first place.

I had to extract out typedefs.h from webrtc_common to resolve
another circular dependency. I believe checks includes typedefs,
but common depends on checks.

Bug: webrtc:7745
Change-Id: I725d49616b1ec0cdc8b74be7c078f7a4d46f084b
Reviewed-on: https://webrtc-review.googlesource.com/33001
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21295}

modules/rtp_rtcp/include/rtp_rtcp_defines.cc

@@ -12,6 +12,27 @@
 
 namespace webrtc {
 
+StreamDataCounters::StreamDataCounters() : first_packet_time_ms(-1) {}
+
+constexpr size_t StreamId::kMaxSize;
+
+bool StreamId::IsLegalName(rtc::ArrayView<const char> name) {
+  return (name.size() <= kMaxSize && name.size() > 0 &&
+          std::all_of(name.data(), name.data() + name.size(), isalnum));
+}
+
+void StreamId::Set(const char* data, size_t size) {
+  // If |data| contains \0, the stream id size might become less than |size|.
+  RTC_CHECK_LE(size, kMaxSize);
+  memcpy(value_, data, size);
+  if (size < kMaxSize)
+    value_[size] = 0;
+}
+
+// StreamId is used as member of RTPHeader that is sometimes copied with memcpy
+// and thus assume trivial destructibility.
+static_assert(std::is_trivially_destructible<StreamId>::value, "");
+
 PayloadUnion::PayloadUnion(const AudioPayload& payload)
     : audio_payload_(payload) {}
 PayloadUnion::PayloadUnion(const VideoPayload& payload)