Added incident report for variations seed signature mismatch.

Added incident report for variations seed signature mismatch.

- Keep record of a bad signature in VariationsSeedStore::LoadSeed.
- Register VerifyVariationsSeedSignature in
  SafeBrowsingService::RegisterAllDelayedAnalysis to send an incident
  report, if a bad signature is encountered.

Originally by georgesak@chromium.org in
https://codereview.chromium.org/646733002/.

BUG=423467
TBR=asvitkine@chromium.org,mattm@chromium.org

Committed: https://crrev.com/7d52139ca255b4985cc61c72abb79ea6ec7476de
Cr-Commit-Position: refs/heads/master@{#308370}

[grt (UTC plus 2), 2014-12-13 14:35:06]

grt@chromium.org changed reviewers:
+ asvitkine@chromium.org, georgesak@chromium.org, mattm@chromium.org

[grt (UTC plus 2), 2014-12-13 14:36:46]

This resolves the wrong-thread DCHECK problem with the original submission. I'll
land this with TBR after manual testing.

[grt (UTC plus 2), 2014-12-15 16:03:24]

The CQ bit was checked by grt@chromium.org

[commit-bot: I haz the power, 2014-12-15 16:03:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/800263002/20001

[commit-bot: I haz the power, 2014-12-15 18:04:44]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-12-15 18:05:29]

Patchset 2 (id:??) landed as
https://crrev.com/7d52139ca255b4985cc61c72abb79ea6ec7476de
Cr-Commit-Position: refs/heads/master@{#308370}

[Alexei Svitkine (slow), 2015-01-05 18:23:29]

lgtm