Remove AttributeCollection's findIndex(Attr*) by using QualifiedName

The internal Attribute and AttributeCollection need not know anything
about the web-exposed Attr.

Since Attr::qualifiedName() is not just a trivial getter, calling that
outside of the loop is probably a good idea anyway.

Review URL: https://codereview.chromium.org/1467143004

Cr-Commit-Position: refs/heads/master@{#361221}
2 files changed