Add webrtc video_coding dependency to content_renderer.

This dependency is there because rtc_video_decoder_factory.h and
a handful other files include video_codec_interface.h. This header
file was not tracked, so this dependency could fly under the radar.

Once this is done and landed upstream, I plan to move the header
to a smaller more focused target; I will update this dependency
then.

Bug: webrtc:7620
Change-Id: I995b740d2d2f796c5744c9f86194cd56949e83d3
Reviewed-on: https://chromium-review.googlesource.com/847480Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526962}

content/renderer/BUILD.gn

@@ -935,6 +935,7 @@ target(link_target_type, "renderer") {
       "//third_party/webrtc/modules/audio_processing",
       "//third_party/webrtc/modules/audio_processing:audio_processing_statistics",
       "//third_party/webrtc/modules/audio_processing/aec_dump",
+      "//third_party/webrtc/modules/video_coding",
       "//third_party/webrtc/modules/video_coding:webrtc_h264",
       "//third_party/webrtc/p2p:libstunprober",
       "//third_party/webrtc/p2p:rtc_p2p",