Do you have other extensions?
If so, try disabling them all, then enable one by one until you find the culprit.

Quoting https://developer.chrome.com/extensions/webRequest#conflict_resolution:

  Only one extension is allowed to redirect a request or modify a header at a time.
  If more than one extension attempts to modify the request,
  the most recently installed extension wins and all others are ignored.
  An extension is not notified if its instruction to modify or redirect has been ignored.

I tried to disable all extensions, even uninstalled them all, but I still got the same issue.

Tried testing the issue on chrome reported version# 73.0.3638.0 using Windows-10 with steps mentioned below:
1) Launched chrome reported version and installed extension from chrome://extensions
2) Opened New Tab Page > Devtools > Network tab and navigated to website(ex: fb.com)
3) Able to see requests generated in network tab(find attahced for the same)

@Reporter: Please find the attached screencast for your reference and let us know if we missed anything in reproducing the issue. If possible provide the sample extension and screencast of the issue which help in better understanding and further triaging it in better way.

Thanks!

Deleted: 914359.mp4 5.2 MB

	914359.mp4 5.2 MB View Download

Screencast file is too large, and I uploaded it to YouTube.
Here is the private link: https://youtu.be/SHA-HBrlMmo

Thank you for providing more feedback. Adding the requester to the cc list.

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

@reporter: Could you please provide a sample extension file as requested in comment#4, which was used in screencast provided in comment#5 so that it would be really helpful in further triaging of the issue.

Thanks.!

The sample extension file used in screencast provided in comment#5:

Deleted: ChromeExtensions.tar.gz 471 bytes

ChromeExtensions.tar.gz 471 bytes Download

Thank you for providing more feedback. Adding the requester to the cc list.

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

Do you have the Network Service enabled?
Does the problem reproduce if you disable it from chrome://flags/#network-service ?

The status of Network Service is the default.

I found that when I upgrade to the latest Canary version, the issue seems like solved. I can successfully modify the value of Content-Type to 'text/plain' in the response headers, and it works, the web page displayed in plain text. But when I open the DevTools, turn to the network tab, the response header displayed in the list was still 'text/html'.

I try to enable the Network Service in chrome://flags, the issue comes again, the modification of response header doesn't work.
Only if I disable the Network Service, or keep it to default, the modification of response header works, but not shown in the network tab.

POC:
chrome.webRequest.onHeadersReceived.addListener(details => {
  details.responseHeaders.find(x => x.name.toLowerCase() === 'content-type').value = 'text/plain';
  return {
    responseHeaders: details.responseHeaders,
  };
}, {urls: ["<all_urls>"]}, ["responseHeaders", "blocking"]);

Thanks for confirming!

Modifying the headers not working with Network Service enabled is  Issue 827582 .

For the solution see https://groups.google.com/a/chromium.org/forum/#!topic/chromium-extensions/vYIaeezZwfQ.

(You need to add the "extraHeaders" option).