Drop const_iterator for AttributeCollection

Drop const_iterator for AttributeCollection and use iterator instead as it is
equivalent. AttributeCollection::iterator is inherently const, while
MutableAttributeCollection::iterator isn't.

Since here the constness is defined by the collection type (AttributeCollection
vs MutableAttributeCollection), I don't think we need to distinguish iterator
vs const_iterator. Using iterator everywhere also makes the code a bit shorter.

R=adamk@chromium.org, esprehn@chromium.org

Review URL: https://codereview.chromium.org/448043003

git-svn-id: svn://svn.chromium.org/blink/trunk@179760 bbb929c8-8fbe-4397-9dbb-9b2b20218538