Convert native handles to buffers before encoding.

Required to permit conversion of NV12 handles on iOS to I420 for VP8
software encoding, which blocks texture-based capture. This change
enforces that all texture-based input provides a method for converting
native handles to I420 if they are ever used with software encoders that
do not understand the native handles.

BUG=4081
R=emircan@chromium.org, glaznev@webrtc.org, hbos@webrtc.org, magjed@webrtc.org, mflodman@webrtc.org, stefan@webrtc.org, tkchin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/50909005

Cr-Commit-Position: refs/heads/master@{#9347}

webrtc/modules/video_coding/main/source/generic_encoder.h

@@ -138,6 +138,8 @@ public:
 
     void OnDroppedFrame();
 
+    bool SupportsNativeHandle() const;
+
 private:
     VideoEncoder* const encoder_;
     VideoEncoderRateObserver* const rate_observer_;