Gamepad: make page visibility behavior layout testable

This CL refactors testing infrastructure for gamepad so that we can
actually write a layout test that would fail if Blink doesn't honor
visibility state. Now Platform::setGamepadListener is now hooked to
GamepadController.

BUG=344556

Review URL: https://codereview.chromium.org/304403002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278723 0039d316-1c4b-4281-b951-d872f2087c98
14 files changed