mac: Fix screen capture for whole-desktop capture.
mac: Fix screen capture for whole-desktop capture.

DisplayStream refresh rects are in display coordinates. When the whole screen is
being captured, the coordinates passed to the ScreenCapturerHelper need to be in
screen coordinates. This CL translates display coordinates to screen
coordinates for whole screen capture.

BUG=chromium:699672

Review-Url: https://codereview.webrtc.org/2740823002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17153}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 31bbee73a029376ca5a6f74e3d5bf224053a6208

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