Clear DesktopFrame in DxgiFrame to avoid legacy image
Clear DesktopFrame in DxgiFrame to avoid legacy image

Once the buffer returned by Windows is not newly allocated, it may contain
legacy images from previous capturing attempts. This usually is not a problem,
as implementations other than ScreenCapturerWinDirectx paint each pixel on the
frame. But due to the one capturer per monitor design of
ScreenCapturerWinDirectx, part of the frame may not be covered by any
DxgiOutputDuplicator, and cause the legacy image to be shown.
So a very simple fix is to clear the DesktopFrame in DxgiFrame.

BUG=708766

Review-Url: https://codereview.webrtc.org/2827983007
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17847}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc