Remove support for the keygen tag

This removes support for <keygen> by updating it
to be an HTMLUnknownElement. As a result, it's
no longer a form-associated element and no
longer has IDL-assigned properties.

The <keygen> tag is still left in the parser,
similar to <applet>, so that it maintains the
document parse behaviours (such as self-closing),
but is otherwise a neutered element.

Tests that were relying on <keygen> having its
own browser-created shadow root (for its custom
select element) have been updated to use
progress bars, while other tests (such as
<keygen>-related crash tests) have been
fully removed.

As Blink no longer treats this tag as special,
all the related IPC infrastructure is removed,
including preferences and enterprise flags,
and all localized strings, as they're all now
unreachable.

This concludes the "Intent to Remove" thread
for <keygen> at
https://groups.google.com/a/chromium.org/d/msg/blink-dev/z_qEpmzzKh8/BH-lkwdgBAAJ

BUG=568184
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2536993002
Cr-Commit-Position: refs/heads/master@{#440245}
249 files changed