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It doesn't really make sense to get overflow scrollbars for an iframe (the
real scrollbars are drawn within the frame, and the frame itself cannot
overflow in any useful way).

Rather than address this in the legacy compositing code, this makes the style
system prevent this nonsensical state from ever occurring, by adjusting the
overflow property to visible for all frame elements.

This matches the existing behavior of the non-composited path here: no
scrollbars are drawn in response to the 'overflow' property on a frame.

BUG=384154
TEST=LayoutTests/scrollbars/iframe-overflow-scroll-ignored.html

Review URL: https://codereview.chromium.org/1843723002

Cr-Commit-Position: refs/heads/master@{#383855}