Fix a crash in Content Settings PolicyProvider::UpdateManagedDefaultSetting

Fix a crash in Content Settings PolicyProvider::UpdateManagedDefaultSetting

This crash was introduced in
https://chromium.googlesource.com/chromium/src/+/26e1b6d4e1d7e27840055e3091520a43f59dde41
where we began checking if managed default settings were valid before
loading them from the policy. However, not all content settings that
can have managed defaults are registered on every platform. So we need
to check that the setting is registered before checking if it is valid.

BUG=721769
Review-Url: https://codereview.chromium.org/2882883002
Cr-Commit-Position: refs/heads/master@{#471974}
Committed: https://chromium.googlesource.com/chromium/src/+/110620d3ae89e65a86ab1afd6149f6f6aa3c6d0c

[raymes, 2017-05-14 22:28:01]

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

[commit-bot: I haz the power, 2017-05-14 22:28:13]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[raymes, 2017-05-14 22:28:32]

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

[commit-bot: I haz the power, 2017-05-14 23:16:06]

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

[commit-bot: I haz the power, 2017-05-14 23:16:07]

Dry run: This issue passed the CQ dry run.

[msramek, 2017-05-15 08:42:20]

LGTM

[raymes, 2017-05-15 22:36:01]

The CQ bit was checked by raymes@chromium.org

[commit-bot: I haz the power, 2017-05-15 22:37:20]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-16 01:36:32]

CQ is committing da patch.

Bot data: {"patchset_id": 20001, "attempt_start_ts": 1494887761012230,
"parent_rev": "c97b0b391db3ec87fbcf5c977aa3369e10f68ad7", "commit_rev":
"110620d3ae89e65a86ab1afd6149f6f6aa3c6d0c"}

[commit-bot: I haz the power, 2017-05-16 01:36:45]

Description was changed from

==========
Fix a crash in Content Settings PolicyProvider::UpdateManagedDefaultSetting

This crash was introduced in
https://chromium.googlesource.com/chromium/src/+/26e1b6d4e1d7e27840055e309152...
where we began checking if managed default settings were valid before
loading them from the policy. However, not all content settings that
can have managed defaults are registered on every platform. So we need
to check that the setting is registered before checking if it is valid.

BUG=721769
==========

to

==========
Fix a crash in Content Settings PolicyProvider::UpdateManagedDefaultSetting

This crash was introduced in
https://chromium.googlesource.com/chromium/src/+/26e1b6d4e1d7e27840055e309152...
where we began checking if managed default settings were valid before
loading them from the policy. However, not all content settings that
can have managed defaults are registered on every platform. So we need
to check that the setting is registered before checking if it is valid.

BUG=721769

Review-Url: https://codereview.chromium.org/2882883002
Cr-Commit-Position: refs/heads/master@{#471974}
Committed:
https://chromium.googlesource.com/chromium/src/+/110620d3ae89e65a86ab1afd6149...
==========

[commit-bot: I haz the power, 2017-05-16 01:36:46]

Committed patchset #2 (id:20001) as
https://chromium.googlesource.com/chromium/src/+/110620d3ae89e65a86ab1afd6149...