Save Page Now functionality breaks on crowded pages

[SoftVision-PaulOiegas]

[Affected versions]:

• Nightly 54.0a1
• NoMore404s 1.7.1

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Latest NoMore404s add-on (v1.7.1) was previously installed on a new profile.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisite.
2. Navigate to http://edition.cnn.com/ page.
3. Click the "NoMore404s" button and choose the "Save Page Now" option.
4. Observe page behavior.

[Expected result]:

• Page is successfully saved in web archive and displayed.

[Actual result]:

• A white blank page is wrongly displayed on the web archive after a long period of loading.

[Notes]:

• The tab correctly reflects the saved page even if the page content is not displayed.
• If the "Save Page Now" breakage occurs, "Recent Version" functionality is not working also since the latest saved page is broken (See logs - https://goo.gl/umghRa)
• Here is a screen recording of the issue - https://goo.gl/xS3MjY.
• Here are the console errors