Clear DesktopFrame in DxgiFrame to avoid legacy image

webrtc/modules/desktop_capture/win/dxgi_frame.cc

Index: webrtc/modules/desktop_capture/win/dxgi_frame.cc
diff --git a/webrtc/modules/desktop_capture/win/dxgi_frame.cc b/webrtc/modules/desktop_capture/win/dxgi_frame.cc
index 0ba678dd0ad21d24831fbff0a87dc24eebd8e3b6..e6bda29bf987cf7fbf9603f081a0052d70bafb4b 100644
--- a/webrtc/modules/desktop_capture/win/dxgi_frame.cc
+++ b/webrtc/modules/desktop_capture/win/dxgi_frame.cc
@@ -40,6 +40,11 @@ bool DxgiFrame::Prepare(DesktopSize size, DesktopCapturer::SourceId source_id) {
     std::unique_ptr<DesktopFrame> frame;
     if (factory_) {
       frame = SharedMemoryDesktopFrame::Create(size, factory_);
+      // DirectX capturer won't paint each pixel in the frame due to its one
+      // capturer per monitor design. So once the new frame is created from
+      // shared memory, we should clear it to avoid the legacy image to be
+      // remained on it. See http://crbug.com/708766.
+      memset(frame->data(), 0, frame->stride() * frame->size().height());

[Sergey Ulanov, 2017/04/20 22:01:10]

I don't think it matters if it's a sharedmemory or not. There is nothing in
BasicDesktopFrame that would zero the buffer. The frame needs to be cleared out
in both cases.

Also this is safe only when stride == width*kBytesPerPixel. It's always the case
for SharedMemoryDesktopFrame and BasicDesktopFrame, but may change, so maybe add
a DCHECK?

[Hzj_jie, 2017/04/20 22:48:58]

Sorry, I always under the impression that we are using default initialization in
BasicDesktopFrame, but indeed not.
https://cs.chromium.org/chromium/src/third_party/webrtc/modules/desktop_captu...

I have updated the code.

     } else {
       frame.reset(new BasicDesktopFrame(size));
     }