[Reland] WebP: Regression test for incremental decoding

Add a test for http://crrev.com/246773006

[Difference from http://crrev.com/255523004: Use hand-created test image].

TBR=tkent@chromium.org 
BUG=364830

Review URL: https://codereview.chromium.org/252923004

git-svn-id: svn://svn.chromium.org/blink/trunk@172837 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Avoid n^2 when attaching a <ul> or <ol>

updateListMarkerNumbers() is called during ::attach() which is going from
firstChild -> lastChild, and which calls nextListItem which calls
NodeRenderingTraversal::nextSibling which also walks from firstChild ->
lastChild producing n^2 traversals over the siblings when a new list is
appended.

This patch adds a guard just like in RenderTreeBuilder. It doesn't fix
the case where you append thousands of <li> after the <ul> has already
been displayed, but this is at least a start.

It makes toggling display from none to block on the body on Wikipedia
50% faster, reducing the attach/detach from 140ms to 70ms on my Mac Pro.

In real world tests it reduces the amount of time spent in recalcStyle
on Wikipedia during page load by 6%, making nextListItem fall off the
profile.

I also removed an incorrect comment about enclosingList returning a
ShadowRoot, that's impossible since it uses NodeRenderingTraversal::parent.
It's also not possible for it to be null.

BUG=366985

Review URL: https://codereview.chromium.org/260223002

git-svn-id: svn://svn.chromium.org/blink/trunk@172838 bbb929c8-8fbe-4397-9dbb-9b2b20218538