[iOS] Fix ios_chrome_unittests on iPad

[iOS] Fix ios_chrome_unittests on iPad

-[SnapshotCache handleLowMemory] DCHECK on iPad so do not call the method
if IsIPadIdiom() returns YES.

BUG=None

Committed: https://crrev.com/20187ac580b052c20961e8ad77ea6163ae9170cb
Cr-Commit-Position: refs/heads/master@{#330747}

[sdefresne, 2015-05-20 16:17:39]

sdefresne@chromium.org changed reviewers:
+ noyau@chromium.org

[sdefresne, 2015-05-20 16:17:39]

PTAL

[noyau (Ping after 24h), 2015-05-20 16:17:59]

The CQ bit was checked by noyau@chromium.org

[noyau (Ping after 24h), 2015-05-20 16:17:59]

lgtm

[commit-bot: I haz the power, 2015-05-20 16:18:29]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1148873002/1

[commit-bot: I haz the power, 2015-05-20 16:24:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-20 16:25:14]

Patchset 1 (id:??) landed as
https://crrev.com/20187ac580b052c20961e8ad77ea6163ae9170cb
Cr-Commit-Position: refs/heads/master@{#330747}