Removing VCMCodecDataBase::Codec and VideoCodingModule::Codec.

This CL brings us one step closer to removing CodecDatabase and
GenericEncoder, by removing the static VCM::Codec(). Codec specific
methods are moved to video_encoder.cc (they already belonged to this
class) and getting default generic codec settings has been moved to a
test specific file.

This CL also makes video_encoder.h pass style guide and lint checks,
since these checks are triggered with the new video_encoder.cc file.

BUG=webrtc:8064

Review-Url: https://codereview.webrtc.org/2993923002
Cr-Commit-Position: refs/heads/master@{#19303}

webrtc/modules/video_coding/generic_encoder_unittest.cc

@@ -32,7 +32,7 @@ class FakeEncodedImageCallback : public EncodedImageCallback {
                         const CodecSpecificInfo* codec_specific_info,
                         const RTPFragmentationHeader* fragmentation) override {
     last_frame_was_timing_ = encoded_image.timing_.is_timing_frame;
-    return Result::OK;
+    return Result(Result::OK);
   };
 
   bool WasTimingFrame() { return last_frame_was_timing_; }