Fix repaint rect reporting in LayoutTests

Currently we dump the raw list of repaint rects in the layer tree dump,
but this is brittle. Any change in the order of repaints, etc, will
cause a diff. This cl changes things so that we sort the repaint rects
before dumping.

We'd retained the raw repaint rects because at the time it seemed
useful to be able to track the order and number of invalidations, but
this has turned out to be more trouble than it's worth.

BUG=371651

Review URL: https://codereview.chromium.org/254633004

git-svn-id: svn://svn.chromium.org/blink/trunk@173990 bbb929c8-8fbe-4397-9dbb-9b2b20218538