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/audio_coding/neteq/include/neteq.h

@@ -18,6 +18,7 @@
 
 #include "api/audio_codecs/audio_decoder.h"
 #include "api/optional.h"
+#include "api/rtp_headers.h"
 #include "common_types.h"  // NOLINT(build/include)
 #include "modules/audio_coding/neteq/neteq_decoder_enum.h"
 #include "rtc_base/constructormagic.h"