Give explicit value to EnabledAtRuntime (IDL extended attribute)

Several IDL extended attributes take an optional value, falling back
to various defaults if the value is absent.
In practice, other than Reflect, the value is almost always used.
The fallback makes the code generator substantially more complicated,
and the behavior opaque, so instead we're making the value required (if possible).

Implementing this is just filling in the value explicitly in the few
places where it's not given, then removing the fallback from the code
generator and updating tests.

I'm starting with EnabledAtRuntime, which this CL fixes.

Review URL: https://chromiumcodereview.appspot.com/23667002

git-svn-id: svn://svn.chromium.org/blink/trunk@156831 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Refactoring: Add Event::createCancelable, and use it.

This CL improves code readability.
This is a preparation to remove Event::create(const AtomicString&, bool, bool).

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23680002

git-svn-id: svn://svn.chromium.org/blink/trunk@156832 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Refactoring: Add toHTMLOListElement, and use it.

We'd like to avoid static_cast.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23582007

git-svn-id: svn://svn.chromium.org/blink/trunk@156833 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce WebSocketHandle and WebSocketHandleClient.

Introduce WebSocketHandle and WebSocketHandleClient in public/platform.
They are interface classes for the new WebSocket implementation.

This CL does not change any behavior.

BUG=275459
R=tyoshino, abarth

Review URL: https://chromiumcodereview.appspot.com/23684002

git-svn-id: svn://svn.chromium.org/blink/trunk@156834 bbb929c8-8fbe-4397-9dbb-9b2b20218538