[CSS Regions] Add performance test for selection

Patch already landed in WebKit. See http://trac.webkit.org/changeset/159488

Add new performance test for selection in CSS Regions. It checks a selection
from the first region to the last one, passing through all the regions.

Test is skipped for now while implementation of selection in CSS Regions is
still evolving.

BUG=319305

Review URL: https://codereview.chromium.org/76293002

git-svn-id: svn://svn.chromium.org/blink/trunk@162418 bbb929c8-8fbe-4397-9dbb-9b2b20218538