[Offline pages] Fixing a crash in URL change, when offline copy not present

[Offline pages] Fixing a crash in URL change, when offline copy not present

Currently code dchecks on the existence of the offline page for a given
bookmark, which might not be the case.
Removing DCHECK and only continuing execution if the offline page exists.

BUG=560518
R=jianli@chromium.org

Committed: https://crrev.com/7278371ad4e9cbceb4b6abf84d43aacb3a8746fb
Cr-Commit-Position: refs/heads/master@{#361363}

[fgorski, 2015-11-23 23:53:20]

PTAL

[jianli, 2015-11-23 23:57:00]

lgtm

[fgorski, 2015-11-23 23:58:43]

The CQ bit was checked by fgorski@chromium.org

[commit-bot: I haz the power, 2015-11-24 00:01:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1473043002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1473043002/20001

[commit-bot: I haz the power, 2015-11-24 02:48:54]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-24 02:48:55]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[fgorski, 2015-11-24 04:01:37]

The CQ bit was checked by fgorski@chromium.org

[commit-bot: I haz the power, 2015-11-24 04:03:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1473043002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1473043002/20001

[commit-bot: I haz the power, 2015-11-24 05:22:28]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-24 05:22:29]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[fgorski, 2015-11-24 16:15:40]

The CQ bit was checked by fgorski@chromium.org

[commit-bot: I haz the power, 2015-11-24 16:17:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1473043002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1473043002/20001

[commit-bot: I haz the power, 2015-11-24 16:40:27]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-11-24 16:41:16]

Patchset 2 (id:??) landed as
https://crrev.com/7278371ad4e9cbceb4b6abf84d43aacb3a8746fb
Cr-Commit-Position: refs/heads/master@{#361363}