Fix issue with DOM Distiller viewer when renderer goes away.

Fix issue with DOM Distiller viewer when renderer goes away.

If the user navigates away from a distilled web page or the
renderer crashes while the DomDistillerViewerSource is still
active, a request can still come in. However, using the
render_process_id and render_frame_id to find the live renderer
(RenderFrameHost) might fail.

That case used to be a DCHECK, but since this happens in the
wild and is reproducible, this CL removes the DCHECK and instead
just bails out early if that happens, since there is no renderer
to send the data to.

It also fixes another place in the same method where the
RenderFrameHost used to be looked up again, but now the one that
has been already found is just reused.

BUG=421949
Committed: https://crrev.com/375f7364c3f315c394d3b7accbd7e236b7ea7b04
Cr-Commit-Position: refs/heads/master@{#302481}

[nyquist, 2014-10-31 21:32:49]

nyquist@chromium.org changed reviewers:
+ cjhopman@chromium.org

[nyquist, 2014-10-31 21:32:58]

cjhopman: PTAL

[cjhopman, 2014-10-31 21:45:53]

lgtm

[nyquist, 2014-11-03 20:36:28]

The CQ bit was checked by nyquist@chromium.org

[commit-bot: I haz the power, 2014-11-03 20:38:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/693053002/1

[commit-bot: I haz the power, 2014-11-03 21:22:38]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-03 21:23:35]

Patchset 1 (id:??) landed as
https://crrev.com/375f7364c3f315c394d3b7accbd7e236b7ea7b04
Cr-Commit-Position: refs/heads/master@{#302481}