Avoid transiently creating disallowed elements when building <use> trees

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=194421

[fs, 2015-04-24 10:20:02]

fs@opera.com changed reviewers:
+ kouhei@chromium.org, pdr@chromium.org

[pdr., 2015-04-24 22:35:42]

The CQ bit was checked by pdr@chromium.org

[pdr., 2015-04-24 22:35:42]

lgtm

LGTM

[commit-bot: I haz the power, 2015-04-24 22:36:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1105873002/1

[commit-bot: I haz the power, 2015-04-25 01:10:51]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=194421

[kouhei (in TOK), 2015-04-25 03:45:43]

lgtm Thanks!