Enable clang style plugin in webrtc/modules/desktop_capture

Enabled the plugin and cleaned up all issues it found, mainly virtual
destructors not being marked as override.

BUG=webrtc:163

Review-Url: https://codereview.webrtc.org/2436503004
Cr-Original-Commit-Position: refs/heads/master@{#14793}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e183121657fa2daf2985f264cff8dd30cd63b97d
32 files changed