Take useful code out of DCHECKS in mime_handler_view_guest.cc

Take useful code out of DCHECKS in mime_handler_view_guest.cc

A mistake I've made in the past! Not sure if it's possible to test this code
in release builds.

BUG=303491
Committed: https://crrev.com/114290f6ba077e3d434cbe61cfdccbb70d6b5f08
Cr-Commit-Position: refs/heads/master@{#293751}

[raymes, 2014-09-08 04:30:39]

raymes@chromium.org changed reviewers:
+ lazyboy@chromium.org

[lazyboy, 2014-09-08 15:46:00]

On 2014/09/08 04:30:40, raymes wrote:

LGTM

Great catch, thanks.

[lazyboy, 2014-09-08 15:47:00]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2014-09-08 15:49:26]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/raymes@chromium.org/547283002/1

[commit-bot: I haz the power, 2014-09-08 17:48:18]

Committed patchset #1 (id:1) as 365b7a11ee58ace59089acb2e70a6b3a24f84d75

[commit-bot: I haz the power, 2014-09-10 03:47:05]

Patchset 1 (id:??) landed as
https://crrev.com/114290f6ba077e3d434cbe61cfdccbb70d6b5f08
Cr-Commit-Position: refs/heads/master@{#293751}