Move iframe@seamless behind the "Experimental WebKit Features" flag.

Move iframe@seamless behind the "Experimental WebKit Features" flag.

Previously, seamless was compile-time enabled via WebKit's
ENABLE_IFRAME_SEAMLESS flag. wkrev.com/138983 added a runtime component
to enable the feature. This patch implements the Chromium side of that
change.

Until we're happy with the state of the feature, it will be hidden
behind the "Experimental WebKit Features" flag, which can be enabled
via `about:flags` for experimentation. Among other benefits, this means
that we won't forget to turn it off before each stable release rolls out
the door.

BUG=168558
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=175747

[Mike West, 2013-01-08 14:42:04]

Hi Eric! Would you mind taking a look at the Chromium side of
https://bugs.webkit.org/show_bug.cgi?id=106213, which moves iframe@seamless
behind the "Experimental WebKit Features" flag?

I haven't thrown the patch to the bots yet, as I'm waiting for
http://trac.webkit.org/changeset/138983 to roll into Chrome, but I expect that
to happen in the next day or two. I certainly won't land this until it's been
through the bots.

+jamesr for OWNERS review.

Thanks!

-mike

[jamesr, 2013-01-08 23:24:36]

lgtm if Eric's happy enough with the state of this feature to turn it on for
users with this flag

[eseidel1, 2013-01-08 23:38:36]

LGTM.

The feature almost ended up on in M24.  From chatting with Hyatt, I
suspect Apple plans to ship with it on next time they ship.

It just has some minimal spec deviations which need smoothing over.
It's nearly ready for prime-time.  Mostly it just languished for
months w/o an owner.  Thankfully Mike has stepped up to the plate to
see it home.

On Tue, Jan 8, 2013 at 3:24 PM,  <jamesr@chromium.org> wrote:
> lgtm if Eric's happy enough with the state of this feature to turn it on for
> users with this flag
>
> https://codereview.chromium.org/11797004/

[commit-bot: I haz the power, 2013-01-09 06:58:56]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mkwst@chromium.org/11797004/1

[commit-bot: I haz the power, 2013-01-09 08:47:54]

Change committed as 175747