Remove ScreenCapturer and WindowCapturer

This change removes ScreenCapturer and WindowCapturer from WebRTC.

BUG=webrtc:6513

Review-Url: https://codereview.webrtc.org/2490063002
Cr-Original-Commit-Position: refs/heads/master@{#15033}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 98903d2f5ef420adb343358824b94731b4a00b38
38 files changed