Target.getTargets does not work for headless. I have to http get /json/list before connect to the page.

This might help: in DevToolsHttpHandler::OnWebSocketRequest, it tries to find the target id in agent_host_map_, which is only updated in handling /json/list or /json/new.

Ill have a look

@skyostil: why don't you reuse connection to speak with the target? I.e. not use Target.sendBlah?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/82f29400ec0c56897e7e464cf91af7a1ce86e145

commit 82f29400ec0c56897e7e464cf91af7a1ce86e145
Author: pfeldman <pfeldman@chromium.org>
Date: Mon Apr 24 20:58:02 2017

DevTools: retain DTAH in all the targets to match their life time.

BUG= 704503 
TBR= changes in cast/ reflect changes to the API

Review-Url: https://codereview.chromium.org/2833213002
Cr-Commit-Position: refs/heads/master@{#466759}

[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chrome/browser/android/devtools_manager_delegate_android.cc
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chrome/browser/android/devtools_manager_delegate_android.h
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chrome/browser/android/tab_android.cc
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chrome/browser/android/tab_android.h
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chrome/browser/devtools/device/devtools_device_discovery.cc
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chrome/browser/devtools/device/devtools_device_discovery.h
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chromecast/browser/devtools/cast_devtools_manager_delegate.cc
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chromecast/browser/devtools/cast_devtools_manager_delegate.h
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/chromecast/browser/devtools/cast_devtools_manager_delegate_unittest.cc
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/content/browser/devtools/devtools_agent_host_impl.cc
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/content/browser/devtools/devtools_http_handler.cc
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/content/browser/devtools/devtools_http_handler.h
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/content/public/browser/devtools_agent_host.h
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/content/public/browser/devtools_manager_delegate.cc
[modify] https://crrev.com/82f29400ec0c56897e7e464cf91af7a1ce86e145/content/public/browser/devtools_manager_delegate.h

#3: That's probably more of a question to the original reporter. Maybe we should update our instructions around this? Also, our C++ bindings currently don't work very nicely with this kind of forwarding.

I've tested the fix by pfeldman and it seems to work. I'm going to close this bug as fixed.