Moved backend injection to the content script

[onionymous]

Moved the backend injection logic to the content script. Previously this was done by using chrome.devtools.inspectedWindow.eval():

const source = `
  // the prototype stuff is in case document.createElement has been modified
  (function () {
    var script = document.constructor.prototype.createElement.call(document, 'script');
    script.src = "${scriptName}";
    script.charset = "utf-8";
    document.documentElement.appendChild(script);
    script.parentNode.removeChild(script);
  })()
  `;

  chrome.devtools.inspectedWindow.eval(source, function(response, error)
...

Since this code runs in the context of the inspected page, it is subject to the page's CSP restrictions and will cause a Trusted Types violation. Content scripts, however, are not subject to the page's restrictions and can manipulate the DOM. This change will prevent a TrustedScriptURL violation on clients that have Trusted Types support.

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[sizebot]

No significant bundle size changes to report.

Generated by 🚫 dangerJS against f09854a

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[bvaughn]

I think this change seems safe. At least based on my own testing, I don't see anything that has regressed. I'll land it internally and see if anyone else notices something!

[bvaughn]

This was the only use of the inject.js module, so we can delete that now.

[bvaughn]

Merged via 8f03109