Use IdTargetObserver in SVGUseElement

Use IdTargetObserver in SVGUseElement

Change SVGUseElement to use the observeTarget() helper from
SVGURIReference. Since SVGUseElement has some additional requirements
for its reference management, a more low-level observeTarget() variant
is exposed.
To facilitate this change, clearShadowTree() is renamed to
clearResourceReference(), and the shadow tree tear-down is hoisted out
of it.

BUG=661598
Review-Url: https://codereview.chromium.org/2744613002
Cr-Commit-Position: refs/heads/master@{#455769}
Committed: https://chromium.googlesource.com/chromium/src/+/9b88406d1a87dfeef593c6d1a91beb3a2b696391

[fs, 2017-03-09 12:37:03]

The CQ bit was checked by fs@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2017-03-09 12:37:19]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-03-09 14:03:46]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-03-09 14:03:46]

Dry run: This issue passed the CQ dry run.

[fs, 2017-03-09 14:54:48]

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

[Stephen Chennney, 2017-03-09 15:35:04]

lgtm

[fs, 2017-03-09 16:42:33]

The CQ bit was checked by fs@opera.com

[commit-bot: I haz the power, 2017-03-09 16:43:06]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-03-09 16:47:40]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1489077753781190, "parent_rev":
"e8442a0169c58e21b309b0afd48ba540b804e5ab", "commit_rev":
"9b88406d1a87dfeef593c6d1a91beb3a2b696391"}

[commit-bot: I haz the power, 2017-03-09 16:48:45]

Description was changed from

==========
Use IdTargetObserver in SVGUseElement

Change SVGUseElement to use the observeTarget() helper from
SVGURIReference. Since SVGUseElement has some additional requirements
for its reference management, a more low-level observeTarget() variant
is exposed.
To facilitate this change, clearShadowTree() is renamed to
clearResourceReference(), and the shadow tree tear-down is hoisted out
of it.

BUG=661598
==========

to

==========
Use IdTargetObserver in SVGUseElement

Change SVGUseElement to use the observeTarget() helper from
SVGURIReference. Since SVGUseElement has some additional requirements
for its reference management, a more low-level observeTarget() variant
is exposed.
To facilitate this change, clearShadowTree() is renamed to
clearResourceReference(), and the shadow tree tear-down is hoisted out
of it.

BUG=661598

Review-Url: https://codereview.chromium.org/2744613002
Cr-Commit-Position: refs/heads/master@{#455769}
Committed:
https://chromium.googlesource.com/chromium/src/+/9b88406d1a87dfeef593c6d1a91b...
==========

[commit-bot: I haz the power, 2017-03-09 16:48:46]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/9b88406d1a87dfeef593c6d1a91b...