Not all Top Sites and Highlights show their correct image preview when dragged

[SoftVision-CiprianMuresan]

https://bugzilla.mozilla.org/show_bug.cgi?id=1415907

[Affected versions]:

• Firefox 58.0a1 Build ID 20171108110838

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a clean new profile.
• Have a sync account with enough History and Bookmarks to populate the sections.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and sync your history.
2. Restart the browser and open a new tab.
3. Wait for Top Sites to load images.
4. Drag all the Top Sites and observe their previews.

[Expected results]:

• The Top Site images are shown as a drag previews.

[Actual results]:

• Some Top Sites show a white preview.

[Notes]:

• The issue is also reproducible when dragging Highlights.
• If you drag the tiles a second time they will show their correct image preview.
• Attached a screen recording of the issue:
  

[Mardak]

Moved to https://bugzilla.mozilla.org/show_bug.cgi?id=1415907