Make sure calling moveParagaph() with proper parameters in InsertListCommand::listifyParagraph()

This patch changes InsertListCommand::listifyParagraph() to calculate start and
end positions of paragraph to move before calling moveParagaph().

Before this patch start and end positions are calculated then we insert a list
element, UL/OL, and list item element, LI. These insertion can make start and
end position no longer points paragraph boundary, attached test case causes this
situation.

Calculating start and end positions of paragraph after all DOM tree
modifications ensures we call moveParagraph with valid parameters.

BUG=351301
TEST=LayoutTests/editing/execCommand/listify-output-crash.html

Review URL: https://codereview.chromium.org/311593003

git-svn-id: svn://svn.chromium.org/blink/trunk@175338 bbb929c8-8fbe-4397-9dbb-9b2b20218538