accessKey should be compared case-insensitively.

This fixes a regression from r153757. I overlooked CaseFoldingHash specified to
HashMap<> included in the previous implementation, which caused capital
accessKeys to be totally ignored.

BUG=303204
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/48433002

git-svn-id: svn://svn.chromium.org/blink/trunk@160771 bbb929c8-8fbe-4397-9dbb-9b2b20218538