Prevent multiple simultaneous calls to GetProof

Prevent multiple simultaneous calls to GetProof

If a malicious or buggy client sends a bunch of CHLOs close together, we
can end up in the following pathological situation:

- The first CHLO is processed in
QuicCryptoServerStream::OnHandshakeMessage, and triggers a call to
ValidateClientHello.
- The ValidateClientHello completes into
QuicCryptoServerStream::FinishProcessingHandshakeMessage, which kicks
off a call to ProcessClientHello.
- While that's going on, another CHLO arrives and
QuicCryptoServerStream::OnHandshakeMessage is called again, kicking off
another call to ValidateClientHello.

Now we are in a situation where two calls to GetProof can be running at
the same time, which is not allowed.

The fix is to note and enforce that the validate_client_hello_cb_ and
process_client_hello_cb_ fields should not be populated at the same
time.  Now, the second call to OnHandshakeMessage will detect that CHLO
processing is already in progress, and close the connection.

relnote: Avoid a QUIC crash which can be triggered by a malicious or
buggy client, especially when GFE is using Mentat for signing.
Protected by default-enabled flag
quic_reloadable_flag_fix_quic_callback_crash.

Merge internal change: 151698587

BUG=

[Zhongyi Shi, 2017-04-03 19:13:50]

The CQ bit was checked by zhongyi@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-03 19:14:56]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-03 19:27:54]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-03 19:27:54]

Dry run: Try jobs failed on following builders:
  ios-device on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-device/builds...)

[Zhongyi Shi, 2017-04-03 20:55:26]

The CQ bit was checked by zhongyi@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-04-03 20:56:17]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[Zhongyi Shi, 2017-04-03 21:19:18]

zhongyi@chromium.org changed reviewers:
+ rch@chromium.org

[Ryan Hamilton, 2017-04-03 23:05:57]

lgtm

[commit-bot: I haz the power, 2017-04-03 23:09:20]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-04-03 23:09:21]

Dry run: This issue passed the CQ dry run.