Make root layer DCHECK in RWC::layout less general

Make root layer DCHECK in RWC::layout less general

WebViewImpl always makes a root layer in layout, but not all
RenderWidgets do.  Instead, only DCHECK if there's a readback
request.

R=jamesr@chromium.org
BUG=397321, 419644
Committed: https://crrev.com/fd619f2a10f1d0be2111b78dace3f975ab01258b
Cr-Commit-Position: refs/heads/master@{#297880}

[enne (OOO), 2014-10-02 14:42:46]

PTAL

I could alternatively call the output request with nothing when there's no root
layer, but that seems like giving somebody in the future a mysterious failure to
debug rather than an assert.

[danakj, 2014-10-02 15:02:15]

danakj@chromium.org changed reviewers:
+ danakj@chromium.org

[danakj, 2014-10-02 15:02:15]

LGTM

[enne (OOO), 2014-10-02 15:04:45]

More specifically, the issue is that the Pepper WebWidget doesn't always have a
root layer after layout in the same way that WebViewImpl does.  It sets and
unsets it separately.  This assert would trigger when it unset it, caused a
commit, and then a layout, and no root layer.

[enne (OOO), 2014-10-02 18:12:41]

cq?

[jamesr, 2014-10-02 18:25:27]

The CQ bit was checked by jamesr@chromium.org

[jamesr, 2014-10-02 18:25:27]

lgtm

[jamesr, 2014-10-02 18:25:46]

It'd be great to make the pepper widget behave more similarly to other
RenderWidgets, of course.

[commit-bot: I haz the power, 2014-10-02 18:26:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/622823002/1

[commit-bot: I haz the power, 2014-10-02 19:27:50]

Committed patchset #1 (id:1) as 76fcfdfa01b5708a953d735f385c14dbf9c4c915

[commit-bot: I haz the power, 2014-10-02 19:28:46]

Patchset 1 (id:??) landed as
https://crrev.com/fd619f2a10f1d0be2111b78dace3f975ab01258b
Cr-Commit-Position: refs/heads/master@{#297880}

[Noel Gordon, 2014-10-03 02:30:16]

On 2014/10/02 18:25:46, jamesr wrote:
> It'd be great to make the pepper widget behave more similarly to other
> RenderWidgets, of course.

@dmichael  maybe file a bug about that?

[dmichael(do not use this one), 2014-10-03 16:26:02]

On 2014/10/03 02:30:16, Noel Gordon wrote:
> On 2014/10/02 18:25:46, jamesr wrote:
> > It'd be great to make the pepper widget behave more similarly to other
> > RenderWidgets, of course.
> 
> @dmichael  maybe file a bug about that?

Done. Feel free to comment.
https://code.google.com/p/chromium/issues/detail?id=420089