Grey placeholders are no longer displayed after importing History

[SoftVision-CiprianMuresan]

https://bugzilla.mozilla.org/show_bug.cgi?id=1405324

[Affected versions]:

• Firefox 58.0a1 Build ID 20171003100226

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Steps to reproduce]:

1. Create a new profile and start the browser with it.
2. Import History from another browser.
3. Observe the Highlights section.

[Expected results]:

• Placeholder images are displayed on the imported Highlights.

[Actual results]:

• Placeholder images are no longer displayed.

[Regression Window]:
Last good revision: 8ce27eeef7662c04deed1328ff25937e530aaf79
First bad revision: e3221268f8fc67bd461c2e04ed486e264c070abf
Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=8ce27eeef7662c04deed1328ff25937e530aaf79&tochange=e3221268f8fc67bd461c2e04ed486e264c070abf

[Notes]:

• Attached a screen recording of the issue:
  

[Mardak]

This is by design now that highlights will use a screenshot if there's no preview image. There were perceived performance concerns of showing the dark placeholder then switching to an image.