Fix a document memory leak in drag-and-drop.

TEST=fast/events/drag-leak-document.html (run_webkit_tests with --enable-leak-detection)
BUG=364392

Review URL: https://codereview.chromium.org/302133002

git-svn-id: svn://svn.chromium.org/blink/trunk@175257 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Don't call moveParagraph() with null |VisiblePositon|

This patch changes |IndentOutdentCommand::outdentParagraph()| not to call
|CompositeEditCommand::movePargrap()| with null |VisiblePositon|.

This situation can be happened in attached test case, where
|visibleStartOfParagraph| and |visibleEndOfPargraph| are second TABLE element.
Both |startOfParagraph()| and |endOfParagraph()| return null |VisiblePositon|
for them.

BUG=379058
TEST=editing/execCommand/outdent-collapse-table-crash-2.html

Review URL: https://codereview.chromium.org/305143002

git-svn-id: svn://svn.chromium.org/blink/trunk@175258 bbb929c8-8fbe-4397-9dbb-9b2b20218538