Fix //components/os_crypt to build for iOS with gn.

Fix //components/os_crypt to build for iOS with gn.

Use set_sources_assignment_filter to correctly select the correct
implementation of KeychainPassword and OsCrypt to use on iOS (the
same as on Mac).

BUG=459705
Committed: https://crrev.com/3dcc7c59edd4cd5931d2f41b306211aa9ddb7a84
Cr-Commit-Position: refs/heads/master@{#359558}

[sdefresne, 2015-11-13 14:44:04]

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

[sdefresne, 2015-11-13 14:44:04]

Please take a look.

[blundell, 2015-11-13 15:30:58]

lgtm

[sdefresne, 2015-11-13 16:05:41]

The CQ bit was checked by sdefresne@chromium.org

[commit-bot: I haz the power, 2015-11-13 16:06:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1439203003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1439203003/1

[commit-bot: I haz the power, 2015-11-13 16:10:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-13 16:11:57]

Patchset 1 (id:??) landed as
https://crrev.com/3dcc7c59edd4cd5931d2f41b306211aa9ddb7a84
Cr-Commit-Position: refs/heads/master@{#359558}