[RootLayerScrolls] Fix logic for computing screen distance of iframe

BUG=823365
R=skobes@chromium.org,sclittle@chromium.org

Change-Id: I400bae2f5a35075b06e42a2b2796772c3a0a687f
Reviewed-on: https://chromium-review.googlesource.com/1107164
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568878}
1 file changed