Make sure end position of selection to delete editable

This patch makes sure DeleteSelectionCommand to work on both start and end
positions of selection to delete editable, by adjusting end position to last
editable position of highest editable root from start position and assertion
for start position.

Start position is editable when DeleteSelection command is invoked
document.execCommand. Newly added assertion for start position will catch
commands which call DeleteSelectionCommand with non-editable selection.

BUG=361935
TEST=LayoutTests/editing/deleting/delete-selection-with-non-editable-end-crash.html

Review URL: https://codereview.chromium.org/303263006

git-svn-id: svn://svn.chromium.org/blink/trunk@175336 bbb929c8-8fbe-4397-9dbb-9b2b20218538