Reland of Move video_encoder.h and video_decoder.h to /api and create GN targets for them

webrtc/media/engine/fakewebrtcvideoengine.h

Index: webrtc/media/engine/fakewebrtcvideoengine.h
diff --git a/webrtc/media/engine/fakewebrtcvideoengine.h b/webrtc/media/engine/fakewebrtcvideoengine.h
index 2dce2c503fcf67e284ef0d3d84027a6dc7ffd74a..f95fa7bdd876ef27943c3fdd409e21e83aa1955b 100644
--- a/webrtc/media/engine/fakewebrtcvideoengine.h
+++ b/webrtc/media/engine/fakewebrtcvideoengine.h
@@ -14,7 +14,10 @@
 #include <map>
 #include <set>
 #include <vector>
+#include <string>
 
+#include "webrtc/api/video_codecs/video_decoder.h"
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/base/basictypes.h"
 #include "webrtc/base/criticalsection.h"
 #include "webrtc/base/gunit.h"
@@ -24,8 +27,6 @@
 #include "webrtc/media/engine/webrtcvideodecoderfactory.h"
 #include "webrtc/media/engine/webrtcvideoencoderfactory.h"
 #include "webrtc/modules/video_coding/include/video_error_codes.h"
-#include "webrtc/video_decoder.h"
-#include "webrtc/video_encoder.h"
 
 namespace cricket {
 static const int kEventTimeoutMs = 10000;