Remove block content clips where we can show that they are unnecessary.

This CL:
1. Modifies RenderBlock::computeOverflow to track, in addition to the visual overflow of the element itself, a bounding rect for the visual overflow of its contents. All call sites of addVisualOverflow are modified to declare whether the visual overflow is affected by the contents clip or not.
2. Modifies RenderBox::pushContentsClip to use this rect to determine whether the contents are visually contained by the clip, in which case it omits the clip.

See measurements in my comment #3 on the bug: https://code.google.com/p/chromium/issues/detail?id=238732#c3

BUG=238732
R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/16402019

git-svn-id: svn://svn.chromium.org/blink/trunk@155847 bbb929c8-8fbe-4397-9dbb-9b2b20218538