Lighthouse does not work with a CSP with sandbox without allow-scripts

#### Provide the steps to reproduce
1. Run LH on https://meh.is/

#### What is the current behavior?
Test will time out

Chrome console will print: `Blocked script execution in 'https://meh.is/' because the document's frame is sandboxed and the 'allow-scripts' permission is not set.`

#### What is the expected behavior?
Lighthouse should work despite website CSP since it is part of the browser.

#### Environment Information
* Affected Channels: DevTools, Extension
* Lighthouse version:
* Chrome version: 87.0.4280.88 (Official Build) (64-bit) (cohort: Stable)
* Node.js version:
* Operating System: Windows 10 1909

Start by reproducing the timeout at https://meh.is/ with Lighthouse in the reported Chrome environment, then trace the DevTools and Extension paths involved in running Lighthouse on sandboxed documents. Verify the change by rerunning the reproduction and confirming Lighthouse completes without requiring the allow-scripts permission.

Written by the indexing model from the issue text.