Don't try to move non visible contents in "Indent" command

This patch makes "Indent" command not to try move non visible contents into
block quote.

The root cause of issue 343958 is "Indent" command calls
|moveParagraphWithClones()| with null |VisiblePosition| for start and end of
contents to move. This patch checks them before calling
|moveParagraphWithClones()|.

BUG=343958
TEST=LayoutTests/editing/execCommand/indent-no-visible-contents-crash.html

Review URL: https://codereview.chromium.org/349143002

git-svn-id: svn://svn.chromium.org/blink/trunk@176735 bbb929c8-8fbe-4397-9dbb-9b2b20218538