Thanks for filing bug.

Are you able to grab a net logs so that we could investigate the issue? Instructions are available at https://dev.chromium.org/for-testers/providing-network-details. 

Nope, instead I uploaded CL to fix it. But may also record a net-log.

https://codereview.chromium.org/2642723008/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a027199ed55462ac711f04cc5c23eff59678beb1

commit a027199ed55462ac711f04cc5c23eff59678beb1
Author: baranovich <baranovich@yandex-team.ru>
Date: Wed Jan 25 15:02:13 2017

Allow proxying plaintext websockets over http/2 proxy

Both secure and insecure websockets are proxied using HTTP CONNECT
tunnels. But if secure proxy supports HTTP/2 plaintext websockets won't
work (ERR_NOT_IMPLEMENTED will be returned).

To initiate standard websocket handshake over established tunnel, in
HttpStreamFactoryImpl::Job::DoCreateStream we should follow
if (!using_spdy_) { ... } code branch, otherwise we end up in
SetSpdyHttpStreamOrBidirectionalStreamImpl which returns
ERR_NOT_IMPLEMENTED for websocket requests.

For wss: scheme, everything works, since ProxyClientSocket (which is
HttpProxyClientSocketWrapper) is wrapped with SSLClientSocket
establishing TLS connection over the tunnel,
HttpStreamFactoryImpl::Job::DoInitConnectionComplete works as
expected. But for ws: scheme, |using_ssl_| is false,
|proxy_info_.is_https()| is true and we get into the branch where
|using_spdy_| is set to true (which is fine for non-websocket
requests).

This change adds very cumbersome condition allowing plaintext
websockets requests over HTTP/2 proxies follow the same code path
secure websockets do.

BUG=684681

TEST=HttpNetworkTransactionTest.PlaintextWebsocketOverSpdyProxy

R=mmenke@chromium.org,ricea@chromium.org

Review-Url: https://codereview.chromium.org/2642723008
Cr-Commit-Position: refs/heads/master@{#446020}

[modify] https://crrev.com/a027199ed55462ac711f04cc5c23eff59678beb1/net/http/http_network_transaction_unittest.cc
[modify] https://crrev.com/a027199ed55462ac711f04cc5c23eff59678beb1/net/http/http_stream_factory_impl_job.cc

M56 full stable release is scheduled next week.
Does this fix needs a merge? If yes, please apply Merge-Request-56 label once the fix is verified in canary.

This bug requires manual review: We are only 5 days from stable.
Please contact the milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), gkihumba@(cros), bustamante@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Since this isn't a regression (plaintext websockets previously didn't work via HTTP/2 proxy) let's wait until M57.  We're really late in the cycle on M56 and are only taking high severity regressions/security changes at this point.

https://codereview.chromium.org/2642723008

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c4e94751fb488deafe919a85f95037ac6c30491c

commit c4e94751fb488deafe919a85f95037ac6c30491c
Author: ricea <ricea@chromium.org>
Date: Thu Feb 02 20:34:32 2017

Revert of Allow proxying plaintext websockets over http/2 proxy (patchset #4 id:60001 of https://codereview.chromium.org/2642723008/ )

Reason for revert:
I believe this is causing the crashes in http://crbug.com/685968.

Original issue's description:
> Allow proxying plaintext websockets over http/2 proxy
>
> Both secure and insecure websockets are proxied using HTTP CONNECT
> tunnels. But if secure proxy supports HTTP/2 plaintext websockets won't
> work (ERR_NOT_IMPLEMENTED will be returned).
>
> To initiate standard websocket handshake over established tunnel, in
> HttpStreamFactoryImpl::Job::DoCreateStream we should follow
> if (!using_spdy_) { ... } code branch, otherwise we end up in
> SetSpdyHttpStreamOrBidirectionalStreamImpl which returns
> ERR_NOT_IMPLEMENTED for websocket requests.
>
> For wss: scheme, everything works, since ProxyClientSocket (which is
> HttpProxyClientSocketWrapper) is wrapped with SSLClientSocket
> establishing TLS connection over the tunnel,
> HttpStreamFactoryImpl::Job::DoInitConnectionComplete works as
> expected. But for ws: scheme, |using_ssl_| is false,
> |proxy_info_.is_https()| is true and we get into the branch where
> |using_spdy_| is set to true (which is fine for non-websocket
> requests).
>
> This change adds very cumbersome condition allowing plaintext
> websockets requests over HTTP/2 proxies follow the same code path
> secure websockets do.
>
> BUG=684681
>
> TEST=HttpNetworkTransactionTest.PlaintextWebsocketOverSpdyProxy
>
> R=mmenke@chromium.org,ricea@chromium.org
>
> Review-Url: https://codereview.chromium.org/2642723008
> Cr-Commit-Position: refs/heads/master@{#446020}
> Committed: https://chromium.googlesource.com/chromium/src/+/a027199ed55462ac711f04cc5c23eff59678beb1

TBR=mmenke@chromium.org,bnc@chromium.org,baranovich@yandex-team.ru
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=684681

Review-Url: https://codereview.chromium.org/2669313002
Cr-Commit-Position: refs/heads/master@{#447832}

[modify] https://crrev.com/c4e94751fb488deafe919a85f95037ac6c30491c/net/http/http_network_transaction_unittest.cc
[modify] https://crrev.com/c4e94751fb488deafe919a85f95037ac6c30491c/net/http/http_stream_factory_impl_job.cc

Since the change was reverted, this bug is now present again: standard websockets are broken when using a standard HTTP/2 proxy.

The change here is very simple, so if it is causing a crash, it should be easy to fix.

The change is simple but the crash is not.

One of our users experienced this error in Chrome today for a website that was using secure websockets. However, the website worked fine in Firefox.

The user might have been behind an HTTP/2 proxy.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/420bb2d8f03a42d0c369597c241a32aca989811f

commit 420bb2d8f03a42d0c369597c241a32aca989811f
Author: Bence Béky <bnc@chromium.org>
Date: Fri Mar 30 17:25:26 2018

Add tests for WebSocket over HTTP/2 proxy.

Bug: 684681, 819101
Change-Id: Ifb7c4ee4f91d00e195c6e7e2da4ca65c86def4a7
Reviewed-on: https://chromium-review.googlesource.com/987518
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547199}
[modify] https://crrev.com/420bb2d8f03a42d0c369597c241a32aca989811f/net/spdy/chromium/spdy_network_transaction_unittest.cc