Remove KeyboardEvent.altGraphKey

Added to WebKit in 2003:
http://trac.webkit.org/changeset/5402

But never standardized:
http://www.w3.org/TR/DOM-Level-3-Events/#interface-KeyboardEvent

It is always false for real events in Chromium. Synthetic events can be
created where it is true using initKeyboardEvent, which is also not per
spec.

Ad-hoc testing with document.createEvent('KeyboardEvent').altGraphKey
shows that this attribute is not supported in IE11, Firefox Nightly or
Opera 12.16, i.e. only WebKit ever added this.

All in all, it seems very unlikely that content could depend on this,
and thus the deprecation process is skipped.

BUG= 399451

Review URL: https://codereview.chromium.org/434683002

git-svn-id: svn://svn.chromium.org/blink/trunk@179545 bbb929c8-8fbe-4397-9dbb-9b2b20218538