Fix memory leak and crash when parsing baked-in configs in domain reliability monitor.

Fix memory leak and crash when parsing baked-in configs in domain reliability monitor.

Invalid baked-in configs would be leaked (in FromJSON) and then crash
due to a null deref (in AddContext).

This change also makes two minor improvements:
- pass scoped_refptrs by const ref
- use a StringPiece rather than a std::string when parsing bake-in configs

BUG=460483
R=ttuttle@chromium.org

Committed: https://crrev.com/2792ed71482cd849ebf312fb02c93efec01f62f5
Cr-Commit-Position: refs/heads/master@{#317375}

[grt (UTC plus 2), 2015-02-20 16:06:09]

PTAL

[Deprecated (see juliatuttle), 2015-02-20 19:15:05]

On 2015/02/20 16:06:09, grt wrote:
> PTAL

lgtm.

[grt (UTC plus 2), 2015-02-20 19:36:05]

The CQ bit was checked by grt@chromium.org

[commit-bot: I haz the power, 2015-02-20 19:37:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/945923002/1

[commit-bot: I haz the power, 2015-02-20 19:40:28]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-20 19:41:17]

Patchset 1 (id:??) landed as
https://crrev.com/2792ed71482cd849ebf312fb02c93efec01f62f5
Cr-Commit-Position: refs/heads/master@{#317375}