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}

rtc_tools/BUILD.gn

@@ -157,6 +157,7 @@ if (!build_with_chromium) {
 
     deps = [
       "..:webrtc_common",
+      "../:typedefs",
       "../common_video",
     ]
   }
@@ -379,6 +380,7 @@ if (rtc_include_tests) {
 
       deps = [
         "..:webrtc_common",
+        "../:typedefs",
         "../common_audio",
         "../modules/audio_processing",
         "../modules/audio_processing:audioproc_debug_proto",