Alter DisplayList so that it doesn't hold on to the prior recording's picture

This addresses the lingering issues from r171691 (Switching Blink to use new Skia SkPictureRecorder API - https://codereview.chromium.org/232513004/)

Specifically:

> DisplayList dl;
> 
> pic = dl.getPicture(); // what does this return?

NULL

> dl.endRecording(); // what does this do?

Nothing (other then throw an ASSERT). getPicture at this point will return NULL.

> canvas = dl.beginRecording();
> canvas = dl.beginRecording(); // what does this do/return?

Returns a new canvas that will feed into a new picture.

> pic = dl.getPicture(); // what does this return (during recording)?

NULL

> dl.endRecording();
> pic = dl.getPicture(); // what does this return (after endRecording)?

The new picture.

Review URL: https://codereview.chromium.org/238043005

git-svn-id: svn://svn.chromium.org/blink/trunk@171888 bbb929c8-8fbe-4397-9dbb-9b2b20218538