modules/video_coding refactorings

The main purpose was the interface-> include rename, but other files
were also moved, eliminating the "main" dir.

To avoid breaking downstream, the "interface" directories were copied
into a new "video_coding/include" dir. The old headers got pragma
warnings added about deprecation (a very short deprecation since I plan
to remove them as soon downstream is updated).

Other files also moved:
video_coding/main/source -> video_coding
video_coding/main/test -> video_coding/test

BUG=webrtc:5095
TESTED=Passing compile-trybots with --clobber flag:
git cl try --clobber --bot=win_compile_rel --bot=linux_compile_rel --bot=android_compile_rel --bot=mac_compile_rel --bot=ios_rel --bot=linux_gn_rel --bot=win_x64_gn_rel --bot=mac_x64_gn_rel --bot=android_gn_rel -m tryserver.webrtc

R=stefan@webrtc.org, tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1417283007 .

Cr-Commit-Position: refs/heads/master@{#10694}

webrtc/modules/video_coding/video_coding_test.gypi

@@ -19,16 +19,16 @@
       ],
       'sources': [
         # headers
-        'main/test/receiver_tests.h',
-        'main/test/rtp_player.h',
-        'main/test/vcm_payload_sink_factory.h',
+        'test/receiver_tests.h',
+        'test/rtp_player.h',
+        'test/vcm_payload_sink_factory.h',
 
         # sources
-        'main/test/rtp_player.cc',
-        'main/test/test_util.cc',
-        'main/test/tester_main.cc',
-        'main/test/vcm_payload_sink_factory.cc',
-        'main/test/video_rtp_play.cc',
+        'test/rtp_player.cc',
+        'test/test_util.cc',
+        'test/tester_main.cc',
+        'test/vcm_payload_sink_factory.cc',
+        'test/video_rtp_play.cc',
       ], # sources
     },
   ],