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

BUG=webrtc:5881
# Because PRESUBMIT ignores LINT blacklist for moved files and these
# headers have some not easy to resolve issues.
NOPRESUBMIT=True

Review-Url: https://codereview.webrtc.org/2780943003
Cr-Commit-Position: refs/heads/master@{#17511}

PRESUBMIT.py

@@ -16,6 +16,8 @@ import sys
 # Files and directories that are *skipped* by cpplint in the presubmit script.
 CPPLINT_BLACKLIST = [
   'tools-webrtc',
+  'webrtc/api/video_codecs/video_decoder.h',
+  'webrtc/api/video_codecs/video_encoder.h',
   'webrtc/base',
   'webrtc/examples/objc',
   'webrtc/media',
@@ -38,8 +40,6 @@ CPPLINT_BLACKLIST = [
   'webrtc/call.h',
   'webrtc/common_types.h',
   'webrtc/common_types.cc',
-  'webrtc/video_decoder.h',
-  'webrtc/video_encoder.h',
   'webrtc/video_send_stream.h',
 ]