Summary:
Fixes a potential issue where double scrollbars could appear or entire main contents would be scrollable (when a plugin has a fixed height > window height) as maincontents was made scrollable as well in D22411617 (2ce65c1b3c)
Anyway this is nice clean up that makes our chrome layout more understandable (imho). The PluginContents wrapper is still very sad (used by ReactDevTools and Hermes debugger), but can hopefully be addressed with the sandy cleanup in the future
Reviewed By: nikoant
Differential Revision: D22431807
fbshipit-source-id: 113e8441e0c43a8b764f5cf27615ab93627c3197
0038c9c9bf