Fix SVGUseElement "instance tree loading" status check

Fix SVGUseElement "instance tree loading" status check

SVGUseElement::instanceTreeIsLoading was not checking the loading status
of the immediate <use> targer (the |targetInstance| argument to the
method.) It also wasn't checking the correct element for loading status,
since correspondingUseElement() will return the containing (innermost)
shadow host - which is not the <use> that will carry the relevant
loading status.
Make sure to check the immediate target, and check the loading status of
any <use> elements encountered (instead of attempting to look at the
"corresponding <use>").
Also rewrite the checker method to use Traversal<...>::next instead of
being recursive, and simplify resourceIsStillLoading() some.

BUG=232901
Committed: https://crrev.com/4cb58892e58014f15722c3375c6da072244e56b2
Cr-Commit-Position: refs/heads/master@{#352645}

[fs, 2015-10-06 15:21:19]

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

[pdr., 2015-10-06 17:03:56]

On 2015/10/06 at 15:21:20, fs wrote:
> 

LGTM!

[pdr., 2015-10-06 17:03:58]

The CQ bit was checked by pdr@chromium.org

[commit-bot: I haz the power, 2015-10-06 17:04:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1388093002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1388093002/1

[commit-bot: I haz the power, 2015-10-06 18:11:17]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-06 18:12:07]

Patchset 1 (id:??) landed as
https://crrev.com/4cb58892e58014f15722c3375c6da072244e56b2
Cr-Commit-Position: refs/heads/master@{#352645}