Remove ViECapture usage in VideoSendStream.

Instead a ViECapturer object is allocated and directly operated on. This
additionally exposes ViESharedData to Call to access the module
ProcessThread, moving towards Call ownership of shared resources.

BUG=1695
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/45339004

Cr-Commit-Position: refs/heads/master@{#9119}

talk/media/webrtc/fakewebrtcvideoengine.h

@@ -718,6 +718,7 @@ class FakeWebRtcVideoEngine
   }
   webrtc::ViEChannel* GetChannel(int channel_id) override { return nullptr; }
   webrtc::ViEEncoder* GetEncoder(int channel_id) override { return nullptr; }
+  webrtc::ViESharedData* shared_data() override { return nullptr; }
 
   WEBRTC_FUNC(CreateReceiveChannel, (int& channel, int original_channel)) {
     return CreateChannel(channel, original_channel);