Allow sheets to be attached at a custom offset #4679

Adds a new "setSheetOffset" API to the `dialog` module, which allows you to change the attachment point for sheets on Mac OS X. I put the API on the dialog module, even though Mac OS X requires that the native window hold and return the desired offset.

1. I was originally hoping to make this an argument on the actual dialog.show* calls, but it seems the parameter set is defined in `libchromiumcontent` and I wasn't sure it would be appropriate to add there?

2.  The API could also be on the BrowserWindow (eg `BrowserWindow.setSheetOffset`). I don't have a strong preference, but I think it's more discoverable on the `dialog` module.

atom/browser/native_window_mac.mm

@@ -243,6 +243,12 @@ bool ScopedDisableResize::disable_resize_ = false;
   return NO;
 }
 
+- (NSRect)window:(NSWindow *)window willPositionSheet:(NSWindow *)sheet usingRect:(NSRect)rect {
+  rect.origin.y = window.contentView.frame.size.height - shell_->GetSheetOffset();
+  return rect;
+}
+
+
 @end
 
 @interface AtomNSWindow : NSWindow {