HTMLAreaElement should expose `download` and `rel` attributes.

HTMLAreaElement should define IDL `download` and `rel` attributes for faster access.
Firefox implements both of them, Safari/Edge has only `rel` (WebKit has no `download` yet).

BUG=605552

Review-Url: https://codereview.chromium.org/2050643002
Cr-Commit-Position: refs/heads/master@{#406541}
7 files changed