Introduce PlatformNotification structures and converters

Introduce PlatformNotification structures and converters

Since modules cannot depend on either Mojo or base yet, introduce a new
Notification representation in platform together with a converter between
itself and the notification.mojom file.

It will be trivial to switch the notification module over to use this
structure rather than the Blink API one, but that depends on Chromium
moving over to Mojo for notifications as well.

Also generate Mojo bindings twice for mojom files defined in Blink: once
backed by WTF, once backed by STL. The former are meant to be used by
users within Blink, whereas the latter are meant for outside Blink usages.

I chose to use "wtf" as the variant rather than "blink" because of the
namespacing that's already in place: "blink::mojom::wtf::*" vs.
"blink::mojom::blink::*".

BUG=595685

[Peter Beverloo, 2016-03-17 20:13:00]

peter@chromium.org changed reviewers:
+ esprehn@chromium.org, mvanouwerkerk@chromium.org

[Peter Beverloo, 2016-03-17 20:13:01]

+esprehn for the union of Blink, WTF, STL and Mojo.
+mvanouwerkerk for notifications

This builds on https://codereview.chromium.org/1814813002/

Elliott, I'd like you to tell me whether this is the right layering— I think it
is. The new classes can go away once the module can depend on Mojo and
base::Time().

[esprehn, 2016-03-18 05:01:30]

modules/ can depend on Mojo today, there's a WTF friendly bindings generator.
base::Time will probably not be coming to blink outside platform, you'll want to
keep helpers in platform that deal with that for you. The web represents time as
a double.

[Peter Beverloo, 2016-03-18 18:04:10]

On 2016/03/18 05:01:30, esprehn wrote:
> modules/ can depend on Mojo today, there's a WTF friendly bindings generator.
> base::Time will probably not be coming to blink outside platform, you'll want
to
> keep helpers in platform that deal with that for you. The web represents time
as
> a double.

Good, then we don't need these classes :).

Split off the Mojo generation to https://codereview.chromium.org/1819473002.