Fix uninitialized member in SafeBrowsingDatabaseNew

Fix uninitialized member in SafeBrowsingDatabaseNew.

Switch to using delegating constructors (https://chromium-cpp.appspot.com/) to streamline initialization.

This was probably not an issue in practice as |change_detected_| is only read in UpdateFinished() and is always set in UpdateStarted().

BUG=Uninitialized |change_detected_| member.

Committed: https://crrev.com/7cd21209b80f6fd444e866f2090705b8f2b575f9
Cr-Commit-Position: refs/heads/master@{#305103}

[gab, 2014-11-20 16:44:40]

gab@chromium.org changed reviewers:
+ mattm@chromium.org

[gab, 2014-11-20 16:44:41]

Matt, PTAL and CQ if it LGTY.

Thanks,
Gab

[mattm, 2014-11-20 22:20:55]

The CQ bit was checked by mattm@chromium.org

[mattm, 2014-11-20 22:20:56]

lgtm

[commit-bot: I haz the power, 2014-11-20 22:22:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/745673002/1

[commit-bot: I haz the power, 2014-11-20 23:06:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-20 23:07:42]

Patchset 1 (id:??) landed as
https://crrev.com/7cd21209b80f6fd444e866f2090705b8f2b575f9
Cr-Commit-Position: refs/heads/master@{#305103}