Stop capturing scrollbars in opacity transitions layout test

The layout test LayoutTests/transitions/opacity-transform-transitions-inside-iframe.html captured platform-specific scrollbars, requiring numerous platform-specific test expectations.

R=jsbell

BUG=368571

Review URL: https://codereview.chromium.org/261553003

git-svn-id: svn://svn.chromium.org/blink/trunk@173056 bbb929c8-8fbe-4397-9dbb-9b2b20218538