Clamp in VisualViewport::setScrollPosition.

This avoids situations where the viewport's scroll animator may be
updated with non-clamped positions by ScrollableArea::setScrollPosition
and RootFrameViewport's scrolling may behave incorrectly.

BUG=626315

Review-Url: https://codereview.chromium.org/2138823002
Cr-Commit-Position: refs/heads/master@{#404799}
2 files changed