Properly calculate the localBounds for elements with overflow clipped

Before this CL, we would pick elements that were not really in the viewport
(e.g they belonged to another scroller above the viewport that had overflow:scroll)

BUG=594878

Review-Url: https://codereview.chromium.org/2136323002
Cr-Commit-Position: refs/heads/master@{#405318}
2 files changed