Make SSLClientSessionCacheOpenSSL thread-safe.

Make SSLClientSessionCacheOpenSSL thread-safe.

Although the network stack is single-threaded, it *should* be legal to spin up
a parallel instance of everything on another thread. Until
https://crbug.com/458365 is resolved, the session cache is one of the few
global mutable objects in the network stack, apart from test code. Thus it
needs to be locked.

BUG=476713
Committed: https://crrev.com/84ce239582c58560bb11c731e17be5aeb45da863
Cr-Commit-Position: refs/heads/master@{#325459}

[davidben, 2015-04-13 23:22:10]

davidben@chromium.org changed reviewers:
+ rsleevi@chromium.org

[Ryan Sleevi, 2015-04-16 01:50:51]

LGTM.

The NSS code is also locked, so this shouldn't cause any regressions compared to
that.

[davidben, 2015-04-16 15:29:18]

The CQ bit was checked by davidben@chromium.org

[commit-bot: I haz the power, 2015-04-16 15:29:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1079943006/20001

[commit-bot: I haz the power, 2015-04-16 17:15:46]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-16 17:16:49]

Patchset 2 (id:??) landed as
https://crrev.com/84ce239582c58560bb11c731e17be5aeb45da863
Cr-Commit-Position: refs/heads/master@{#325459}