Add a comment for HTMLInputElement.files attribute

Add a comment for HTMLInputElement.files attribute to explain that
it is intentionally not marked as readonly because of:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22682

NOTRY=true
R=tkent
BUG=263060

Review URL: https://chromiumcodereview.appspot.com/19963003

git-svn-id: svn://svn.chromium.org/blink/trunk@154732 bbb929c8-8fbe-4397-9dbb-9b2b20218538