Add ::Leaky to the LazyInstance<SafeBrowsingServiceFactoryImpl> in

Add ::Leaky to the LazyInstance<SafeBrowsingServiceFactoryImpl> in
safe_browsing_service.cc.  Hopefully this will fix the "pure virtual method
called" errors seen in various ExtensionService tests that use safe browsing;
both those reverted as part of bug 233290, and the ones that are still live.

BUG=233290
R=jyasskin@chromium.org
TBR=shess@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=233426

[not at google - send to devlin, 2013-11-06 23:22:40]

This is Jeffrey's idea for a fix here. Thanks Jeffrey.

[Jeffrey Yasskin, 2013-11-06 23:40:00]

lgtm with the caveat that this is speculative, based on
https://commondatastorage.googleapis.com/chromium-build-logs/logs/2013/11/ahV....
I don't see a great explanation for the appearance that a LazyInstance is being
destroyed during a test execution, and someone with more context might want to
look into that.

[not at google - send to devlin, 2013-11-07 00:10:36]

I'm submitting this now in the hope that it does quash some of those flaky
failures.

[not at google - send to devlin, 2013-11-07 00:10:59]

Committed patchset #1 manually as r233426 (presubmit successful).