I believe this started with crrev.com/414707. Before, extension background pages would get reported with "type":"background_page". Now, I'm seeing "type":"other" for the first few requests to /json, and then it changes to "type":"background_page".

Unfortunately I can only reproduce this on our buildbots, and my MacBook Air, but not my Ubuntu desktop.

Sample logs are below, note the "type" field for the generated background page.

[3.233][DEBUG]: DevTools request: http://localhost:12875/json
[3.374][DEBUG]: DevTools response: [ {
   "description": "",
   "devtoolsFrontendUrl": "/devtools/inspector.html?ws=localhost:12875/devtools/page/adf67969-5a41-4ec3-9bb6-caf8ee598ad4",
   "id": "adf67969-5a41-4ec3-9bb6-caf8ee598ad4",
   "title": "data:,",
   "type": "page",
   "url": "data:,",
   "webSocketDebuggerUrl": "ws://localhost:12875/devtools/page/adf67969-5a41-4ec3-9bb6-caf8ee598ad4"
}, {
   "description": "",
   "devtoolsFrontendUrl": "/devtools/inspector.html?ws=localhost:12875/devtools/page/495bf4ae-888c-41d0-881e-c028ccc0b7be",
   "id": "495bf4ae-888c-41d0-881e-c028ccc0b7be",
   "title": "chrome-extension://aapnijgdinlhnhlmodcfapnahmbfebeb/_generated_background_page.html",
   "type": "other",
   "url": "chrome-extension://aapnijgdinlhnhlmodcfapnahmbfebeb/_generated_background_page.html",
   "webSocketDebuggerUrl": "ws://localhost:12875/devtools/page/495bf4ae-888c-41d0-881e-c028ccc0b7be"
} ]

...and then later...

[6.968][DEBUG]: DevTools request: http://localhost:12875/json
[7.004][DEBUG]: DevTools response: [ {
   "description": "",
   "devtoolsFrontendUrl": "/devtools/inspector.html?ws=localhost:12875/devtools/page/4332e2e8-84ba-4a10-a2d2-9717758e06b8",
   "id": "4332e2e8-84ba-4a10-a2d2-9717758e06b8",
   "title": "http://localhost:4534/common/blank.html",
   "type": "page",
   "url": "http://localhost:4534/common/blank.html",
   "webSocketDebuggerUrl": "ws://localhost:12875/devtools/page/4332e2e8-84ba-4a10-a2d2-9717758e06b8"
}, {
   "description": "",
   "id": "adf67969-5a41-4ec3-9bb6-caf8ee598ad4",
   "title": "Testing Alerts",
   "type": "page",
   "url": "http://localhost:4534/common/alerts.html"
}, {
   "description": "",
   "devtoolsFrontendUrl": "/devtools/inspector.html?ws=localhost:12875/devtools/page/495bf4ae-888c-41d0-881e-c028ccc0b7be",
   "id": "495bf4ae-888c-41d0-881e-c028ccc0b7be",
   "title": "Chrome Automation Extension",
   "type": "background_page",
   "url": "chrome-extension://aapnijgdinlhnhlmodcfapnahmbfebeb/_generated_background_page.html",
   "webSocketDebuggerUrl": "ws://localhost:12875/devtools/page/495bf4ae-888c-41d0-881e-c028ccc0b7be"
} ]