Thumbnails should be a clean image representation, remove frame.

Thumbnails should be a clean image representation without "hard-coded" borders or design elements.

Many thumbnail generators currently return ThumbCreator::DrawFrame in their flags()
function which causes a 90's style 1px "3d" border to be added to the thumbnail image.
This causes problems when consumers of these thumbnails (like Dolphin) expect a
clean image representation of the file, without hard-coded design elements.
It also looks really bad when any resizing/scaling happens or when the thumbnail
is shown on a tooltip. Especially now that the UI has been modernized with Breeze.

REVIEW: 129921