[iOS] Fix a typo in ios_strings_resources.grd

[iOS] Fix a typo in ios_strings_resources.grd

tools/grit/grit.py does not recognize "preprend" as a valid emit_type but
instead understand "prepend" which avoid the inclusion of atlres.h in the
generated file.

BUG=475514
Committed: https://crrev.com/51689d131ef4401d3349684ab466e666469f2c7e
Cr-Commit-Position: refs/heads/master@{#327323}

[sdefresne, 2015-04-28 17:37:31]

The CQ bit was checked by sdefresne@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-04-28 17:37:47]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1113603003/1

[droger, 2015-04-28 17:37:58]

lgtm

[sdefresne, 2015-04-28 17:37:58]

The CQ bit was unchecked by sdefresne@chromium.org

[droger, 2015-04-28 17:38:06]

The CQ bit was checked by droger@chromium.org

[commit-bot: I haz the power, 2015-04-28 17:38:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1113603003/1

[commit-bot: I haz the power, 2015-04-28 18:06:32]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-28 18:07:24]

Patchset 1 (id:??) landed as
https://crrev.com/51689d131ef4401d3349684ab466e666469f2c7e
Cr-Commit-Position: refs/heads/master@{#327323}