Revert "SideBySide2: Center content in the browser window"

This reverts commit 875590301a68377367f14f82fc6f1056a3a17499.

The auto margin left/right and width was making the view too wide for
15" displays, causing the navigation arrows and overview gutter to be
completely cut off.

Change-Id: I0c730a901044344b5027cff91928a8c64c8c599b