Change <dialog> centering to not mutate style.

Previously, <dialog> centering was implemented via
customStyleForRenderer. However, it's non-conforming to adjust the
computed style; the spec mandates that only the static position is
changed. This patch changes centering to occur during layout of
RenderView, which is the parent of all absolutely positioned dialogs.

BUG=232268

Review URL: https://codereview.chromium.org/26667002

git-svn-id: svn://svn.chromium.org/blink/trunk@159767 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Change dynamic srcset test to avoid useless download

In order to avoid useless downloads when dynamically adding a srcset attribute, the attribute's value must include the 1x resource as well.
I've changed the dynamic srcset addition test to reflect that (and stop downloading the 2x resource for nothing)

BUG=306523

Review URL: https://codereview.chromium.org/26849003

git-svn-id: svn://svn.chromium.org/blink/trunk@159768 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Suppress tests that need to be rebaselined after http://crrev.com/24090003 lands

This suppresses ImageOnlyFailures for
virtual/softwarecompositing/culling/filter-occlusion*, which will need to
be rebaselined after Chromium CL http://crrev.com/24090003 lands.

TBR=senorblanco@chromium.org
BUG=160302

Review URL: https://codereview.chromium.org/27174004

git-svn-id: svn://svn.chromium.org/blink/trunk@159769 bbb929c8-8fbe-4397-9dbb-9b2b20218538