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### Issue Details

* **Electron Version:**
  * 7.1.7
* **Operating System:**
  * macOS 10.14.6
* **Last Known Working Electron version:**
  * 6.1.7

### Expected Behavior
When loading a Javascript file using a custom protocol, opening the dev tools should load the source maps.

### Actual Behavior
It does not load the source maps, and produces no error message other than an `ERR_UNKNOWN_URL_SCHEME` in the net log if net logging is enabled.

### To Reproduce
To see it work properly on Electron 6.1.7:
1. Run [this fiddle](https://gist.github.com/d0cb93e9c7d6ad198fe38db77038495a) on Electron 6.1.7
2. Open the dev tools
3. Note that `source map requested!` was logged to the main process console, and the source map for `test.js` (including `test.ts`) was loaded into the dev tools

To see it fail on Electron 7.1.7:
1. Run [this fiddle](https://gist.github.com/d0cb93e9c7d6ad198fe38db77038495a) on Electron 7.1.7
2. Open the dev tools
3. Note that `source map requested!` was not logged to the main process console, and neither `test.js.map` nor `test.ts` were loaded

### Additional Information
This is probably more fallout from Chromium's network service. When I enable net logging (`app.commandLine.appendSwitch('log-net-log', 'netlog.json')`), I see the following when opening the dev tools:

```
{"params":{"priority":"IDLE","traffic_annotation":129652775,"url":"serve://dist/assets/test.js.map"},"phase":1,"source":{"id":27,"type":1},"time":"20404995","type":2},
{"phase":1,"source":{"id":27,"type":1},"time":"20404995","type":103},
{"phase":2,"source":{"id":27,"type":1},"time":"20404995","type":103},
{"params":{"load_flags":0,"method":"GET","privacy_mode":0,"url":"serve://dist/assets/test.js.map"},"phase":1,"source":{"id":27,"type":1},"time":"20404995","type":99},
{"params":{"net_error":-302},"phase":2,"source":{"id":27,"type":1},"time":"20404995","type":99},
{"phase":1,"source":{"id":27,"type":1},"time":"20404995","type":107},
{"phase":2,"source":{"id":27,"type":1},"time":"20404995","type":107},
{"params":{"net_error":-302},"phase":2,"source":{"id":27,"type":1},"time":"20404995","type":2},
```

Note the `"net_error":-302` -- `-302` is `ERR_UNKNOWN_URL_SCHEME`, so somehow the network service(?) isn't aware of the standard scheme that was registered when trying to load `test.js.map`, even though `test.js` was successfully loaded using that same scheme.

This is currently preventing `ember-electron` apps from loading source maps on 7.x since `ember-electron` relies on a custom protocol to load the Ember app from the filesystem while still allowing Ember's web-based tooling (route recognition, web-style absolute asset URLs, etc.) to function.
shell/browser/ui/inspectable_web_contents.cc
bool docked) {
  if (managed_devtools_web_contents_)
    view_->SetIsDocked(docked, activate_);
  if (!callback.is_null())
    std::move(callback).Run(nullptr);
}
void InspectableWebContents::OpenInNewTab(const std::string& url) {}
void InspectableWebContents::ShowItemInFolder(
    const std::string& file_system_path) {
  if (file_system_path.empty())
    return;
  base::FilePath path = base::FilePath::FromUTF8Unsafe(file_system_path);
  
  platform_util::OpenPath(path, platform_util::OpenCallback());
}
void InspectableWebContents::SaveToFile(const std::string& url,
                                        const std::string& content,
                                        bool save_as) {
  if (delegate_)
    delegate_->DevToolsSaveToFile(url, content, save_as);
}
void InspectableWebContents::AppendToFile(const std::string& url,