Modify supercluster multiplier logic.

Previously the multiplier was based on the first root touched by the current
layout pass (for table cells) or the deepest common ancestor of all the roots
(for other superclusters).

We now base the multiplier on the max known width of any root.  This is still
not great since not all widths are known, but it gives us consistent results
on hacker news.

BUG=361749
R=pdr@chromium.org

Review URL: https://codereview.chromium.org/231663006

git-svn-id: svn://svn.chromium.org/blink/trunk@171364 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix KURL::copy such that it also copies m_innerURL when available

BUG=362061

Review URL: https://codereview.chromium.org/234853002

git-svn-id: svn://svn.chromium.org/blink/trunk@171365 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move RenderLayerCompositor trigger caching to be when settings change

m_hasAcceleratedCompositing, m_showRepaintCounter and 
m_compositingTriggers are all just caches of values in Settings.
Instead of updating them after every style recalc and layout,
we should update them when the settings in question change.

Add an invalidation hook to Settings for updating compositing
triggers. Also, remove the hook that forces a style recalc
in acceleratedCompositingEnabled and showRepaintCounter.
As best I can tell, this call doesn't make sense anymore.
Compositing state can affect the results of style recalc,
but we're not actually setting needs recalc on any nodes,
so we wouldn't update them unless they already needed
recalc. I traced this back to
http://src.chromium.org/viewvc/blink?view=revision&revision=45199.

In addition to minimizing the amount of chicken-egg settings
update we have, this enables moving the accelerated fixed position
setting to being a trigger in CompositingReasonFinder
in https://codereview.chromium.org/231613002/. Without this patch,
we incorrectly get the wrong trigger value because we read
the compositing triggers inside of style recalc.

Unfortunately, we still need to set m_forceCompsitingMode in
updateCompositingLayersAfterLayout because layout can affect
whether we believe an iframe's content to be scrollable,
and thus whether it should be force composited.

Review URL: https://codereview.chromium.org/233323004

git-svn-id: svn://svn.chromium.org/blink/trunk@171366 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Set NeedsRebaseline on mock scrollbar changes

These were marked as NeedsManualRebaseline in Blink r171349.  The change
to scrollbars was made in Chromium r263273.  Marking these now as
NeedsRebaseline.

TBR=abarth@chromium.org
BUG=362078

Review URL: https://codereview.chromium.org/235823002

git-svn-id: svn://svn.chromium.org/blink/trunk@171367 bbb929c8-8fbe-4397-9dbb-9b2b20218538