Add DesktopCapturer::Result::MAX_VALUE
Add DesktopCapturer::Result::MAX_VALUE

MAX_VALUE will be used in chromoting_messages.h to define range of
valid DesktopCapturer::Result values.

BUG=webrtc:5950
R=jamiewalch@chromium.org

Review URL: https://codereview.webrtc.org/2069103003 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13157}

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