Fix DesktopAndCursorComposer to restore frames to the original state.

Screen capturers may reuse frame buffers and they expect that the
frame content isn't changed by the frame consumer.
DesktopAndCursorComposer draws mouse cursor on generated frames and
it was releasing the frames with the mouse cursor on them. Fixed
it to restore frame content erasing mouse cursor before returning
desktop frames.

BUG=crbug.com/316297
R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/3899004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5133 4adac7df-926f-26a2-2b94-8c16560cd09d
7 files changed