Overflow Debugging Tool (unwanted scrollbar debugging)

[violasong]

We want to help users with these problems:

1. Finding the container element that scrolls.
2. Finding the elements that cause the container element to scroll.

For step 1, the proposed solution is a new [scroll] inspector badge, as well as the ability to click on scrollbars while picking elements. (This has landed.)

For step 2, the proposed solution is making the [scroll] badge clickable. Clicking the badge would reveal the element(s) that cause the overflow. This could be done by filtering the markup-view so only the container and the overflow-causing elements are shown. (This has been started but needs more design work.)