DevTools: Fix dropdown width in settings after opening profilers panel

BUG=407485

Review URL: https://codereview.chromium.org/500543003

git-svn-id: svn://svn.chromium.org/blink/trunk@180921 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Get rid of computation of the smaller invalidation rect when changing a background image.

Instead just invalidate the containing RenderObject.

This is not such a performance-critical operation that it justifies this code complexity.

Furthermore, it's unclear that it actually has a performance benefit,
given that the image is often the same size or close to it as the containing
RenderObject subclass.

BUG=

Review URL: https://codereview.chromium.org/497233005

git-svn-id: svn://svn.chromium.org/blink/trunk@180922 bbb929c8-8fbe-4397-9dbb-9b2b20218538