Match HI spec for thumbnail view sizing and location

Match HI spec for thumbnail view sizing and location
https://bugs.webkit.org/show_bug.cgi?id=186412
<rdar://problem/40226192>

Reviewed by Tim Horton.

Use the computed obscured inset to position the QuickLook
view inside the WKSystemPreviewView.

* UIProcess/ios/WKSystemPreviewView.mm:
(-[WKSystemPreviewView web_setContentProviderData:suggestedFilename:]):
(-[WKSystemPreviewView _layoutThumbnailView]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@232606 268f45cc-cd09-0410-ab3c-d52691b4dbfc