Fix the problem that merging paragraphs including non-editable element brakes.

This patch was originally commited to WebKit:
http://trac.webkit.org/changeset/157944 by Santosh Mahto

When we merge two paragraphs which later one has non-editable elements by deleting the line end, merged paragraph doesn't contain the non-editable elements and elements after the elements.

This happens because endOfParagraphToMove calculation in merging function stop at editing boundary so endOfParagraphToMove becomes position just before non-editable content.
With this patch now endOfParagraphToMove is calculated by skipping over the non-editable element.

BUG=306970

Review URL: https://codereview.chromium.org/141543004

git-svn-id: svn://svn.chromium.org/blink/trunk@166390 bbb929c8-8fbe-4397-9dbb-9b2b20218538