The newest builds of Chromium feature a Chrome Experiment via “about:flags” that I have not seen before: composited render layer borders. When I first looked at this, I assumed it must have something to do with the increased use of graphical capabilities being added into Chrome, and I was right.
According to the Chromium documentation regarding WebKit rendering basics, a RenderObject is associated with a RenderLayer. The purpose of these layers is to allow content to overlap each other as they are composited. The problem, however, is that oftentimes these layers are either semi-transparent or not visible at all.
This means that for development purposes it is better to be able to see these individual layers via borders that can be overlayed. An example of this can be seen by a developer bug report where this Experiment was accidently turned on showing these borders.
As features get more complex inside the browser tools are required to be able to properly debug problems with applications. Chrome Web Store applications with increasingly complicated graphical features would be a useful reason to use an Experiment like this one.