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}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 592039f..94dd865 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -59,6 +59,8 @@
     "proxy.h",
     "rtcerror.cc",
     "rtcerror.h",
+    "rtp_headers.cc",
+    "rtp_headers.h",
     "rtpparameters.cc",
     "rtpparameters.h",
     "rtpreceiverinterface.h",
@@ -84,6 +86,7 @@
   ]
 
   deps = [
+    ":array_view",
     ":optional",
     ":rtc_stats_api",
     ":video_frame_api",
@@ -93,10 +96,14 @@
     # Basically, don't add stuff here. You might break sensitive downstream
     # targets like pnacl. API should not depend on anything outside of this
     # file, really. All these should arguably go away in time.
+    "..:typedefs",
     "..:webrtc_common",
     "../modules/audio_processing:audio_processing_statistics",
+    "../rtc_base:checks",
+    "../rtc_base:deprecation",
     "../rtc_base:rtc_base",
     "../rtc_base:rtc_base_approved",
+    "../rtc_base:stringutils",
   ]
 
   # This is needed until bugs.webrtc.org/7504 is removed so this target can