[Blink] It is possible to select content outside nested editable

[Piotrek Koszuliński]

Steps to reproduce

1. Open ​http://sdk.ckeditor.com/samples/simplebox.html
2. Triple click in the simple box header.
3. Press backspace.

Expected result

Only content of the header was touched.

Actual result

Text from the simplebox content was moved to the header.

Other details (browser, OS, CKEditor version, installed plugins)

This means that backpace handler is also broken as it crosses editable boundaries: #14354.

NOTE: once #14354 is fixed you will need to investigate where's the selection to confirm the issue.

Reported in ​http://stackoverflow.com/questions/34992972/ckeditor-titlebody-simplebox-example-widget-select-title-text-and-remove