_setDidMoveSwipeSnapshotCallback's block should only be called if the snapshot is...

_setDidMoveSwipeSnapshotCallback's block should only be called if the snapshot is going to move
https://bugs.webkit.org/show_bug.cgi?id=136354
<rdar://problem/18167791>

Reviewed by Simon Fraser.

* UIProcess/API/Cocoa/WKViewPrivate.h:
Update the comment to note the new behavior.

* UIProcess/mac/ViewGestureControllerMac.mm:
(WebKit::ViewGestureController::beginSwipeGesture):
Don't call the block until the snapshot is actually moving (if the snapshot
never moves because it's behind the live view, don't call the block at all).

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@173093 268f45cc-cd09-0410-ab3c-d52691b4dbfc