Merge 194421 "Avoid transiently creating disallowed elements whe..."

Merge 194421 "Avoid transiently creating disallowed elements whe..."

> Avoid transiently creating disallowed elements when building <use> trees
> 
> When building a shadow tree for a <use>, a direct reference to a
> "disallowed" element would cause the element to first be inserted before
> buildShadowTree() noticed it's disallowed and returns false, so it's
> removed again.
> This transient mutation could take place while computing an animation
> update, if a CSS property was being animated and the layout tree/style
> was dirty.
> Avoid the insert-remove sequence by checking if the initial target is
> disallowed up-front. This matches how it's done in the general subtree
> building case inside buildShadowTree().
> 
> BUG=478549
> 
> Review URL: https://codereview.chromium.org/1105873002

TBR=fs@opera.com

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=195031

[fs, 2015-05-07 08:03:51]

Committed patchset #1 (id:1) manually as r195031.