Use more const, more RefPtrs while iterating the SVG shadow tree.

Use more const, more RefPtrs while iterating the SVG shadow tree.

"Weak" iteration with raw pointers can cause bugs such as Issue 225403
if the tree is modified during iteration. This makes SVGUseElement use
RefPtrs when appending children, and const pointers other times, when
iterating over the tree.

Plus some lint fixes.

BUG=225420
Committed: https://crrev.com/7d78afae216dbb4326bdfbe5c806a3801207ea77
Cr-Commit-Position: refs/heads/master@{#351974}

[dominicc (has gone to gerrit), 2015-09-30 05:36:46]

dominicc@chromium.org changed reviewers:
+ kouhei@chromium.org

[dominicc (has gone to gerrit), 2015-09-30 05:36:54]

PTAL

[fs, 2015-09-30 08:47:10]

fs@opera.com changed reviewers:
+ fs@opera.com

[fs, 2015-09-30 08:47:10]

lgtm

(I wonder why we'd allow script execution at all in these contexts though...?)

[kouhei (in TOK), 2015-09-30 09:13:44]

lgtm

[dominicc (has gone to gerrit), 2015-10-02 05:56:22]

The CQ bit was checked by dominicc@chromium.org

[commit-bot: I haz the power, 2015-10-02 05:56:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1377063004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1377063004/1

[commit-bot: I haz the power, 2015-10-02 08:18:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-02 08:19:24]

Patchset 1 (id:??) landed as
https://crrev.com/7d78afae216dbb4326bdfbe5c806a3801207ea77
Cr-Commit-Position: refs/heads/master@{#351974}