Use visibility: hidden; to hide inspectors

Using display: none; on the inspectors WindowSet causes Firefox to
neglect to load CSS in the inspectors' iframes, which means the frame
never initializes and the inspector never opens.

Bug: 57568
Change-Id: Ia1a0ce78754fa1318a7d439abe1f0e2f86420e7a
Merge "Use visibility: hidden; to hide inspectors"