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: refs/heads/master@{#13157}

webrtc/modules/desktop_capture/desktop_capturer.h

@@ -37,6 +37,8 @@ class DesktopCapturer {
     // Capture has failed and will keep failing if the caller tries calling
     // Capture() again.
     ERROR_PERMANENT,
+
+    MAX_VALUE = ERROR_PERMANENT
   };
 
   // Interface that must be implemented by the DesktopCapturer consumers.