[DevTools] Enable multiple sessions

This patch allows:
- multiple connections through http handler;
- multiple debugging sessions via debugger api from different
  extensions, but not from the same one;
- single DevTools window in addition to any other debuggers.

Bug: 590878
Change-Id: Ib19ff0325235a40432f4f247edaefc2dfd5cb2a2
Reviewed-on: https://chromium-review.googlesource.com/629816
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497170}

• chrome/browser/devtools/devtools_ui_bindings.cc[diff]
• chrome/browser/extensions/api/debugger/debugger_api.cc[diff]
• chrome/browser/extensions/api/debugger/debugger_api.h[diff]
• chrome/test/data/extensions/api_test/debugger/background.js[diff]
• content/browser/devtools/devtools_agent_host_impl.cc[diff]
• content/browser/devtools/devtools_agent_host_impl.h[diff]
• content/browser/devtools/devtools_http_handler.cc[diff]
• content/browser/devtools/protocol/target_handler.cc[diff]
• content/browser/devtools/site_per_process_devtools_browsertest.cc[diff]
• content/public/browser/devtools_agent_host.h[diff]
• content/shell/browser/shell_devtools_bindings.cc[diff]
• headless/lib/browser/headless_devtools_client_impl.cc[diff]