Don't move uneditable paragraph contents in CompositeEditCommand::moveParagraphContentsToNewBlockIfNecessary

This patch makes CompositeEditCommand::moveParagraphContentsToNewBlockIfNecessary
not to move uneditable paragraph contents.

In attached test case, Blink tried to move contents in uneditable ABBR element.

BUG=381076, 383811
TEST=LayoutTests/editing/execCommand/justy-center-with-uneditable-crash.html

Review URL: https://codereview.chromium.org/335773002

git-svn-id: svn://svn.chromium.org/blink/trunk@176172 bbb929c8-8fbe-4397-9dbb-9b2b20218538