Delete unneeded dependencies on libjingle_peerconnection_api

Also annotate a few of the remaining uses, to guide further splits of
that large build target.

Bug: webrtc:8733
Change-Id: I16ac33ab48e6d39a1a8dbc2a3fc671d8db6dbfe9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150789
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29001}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index d55fe52..925f32f 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -291,6 +291,7 @@
   ]
 
   deps = [
+    # For api/stats_types.h
     ":libjingle_peerconnection_api",
     ":rtp_parameters",
   ]
@@ -690,6 +691,7 @@
       "test/mock_frame_encryptor.h",
     ]
     deps = [
+      # For api/crypto/frame_encryptor_interface.h
       ":libjingle_peerconnection_api",
       "../test:test_support",
     ]