Get rid of false assert from CompositeEditCommand::moveParagraphContentsToNewBlockIfNecessary

This patch removes false assumption from CompositeEditCommand::moveParagraphContentsToNewBlockIfNecessary,
which attached test case breaks that.

In the test case attached test case, |upstreamStart| points empty UL elemnt,
but it is visble, and |upstreamEnd| points A element. Thus, |upstreamStart|
isn't descendant of |upstreamEnd|. When removing "style" attribute from UL
element to make it invisible, this test case doesn't cause assertion.

BUG=366068
TEST=LayoutTests/editing/execCommand/justify-center-empty-list-crash.html

Review URL: https://codereview.chromium.org/300023009

git-svn-id: svn://svn.chromium.org/blink/trunk@175118 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unimplemented nodeUnderMouse() method from EventHandler.h

nodeUnderMouse function is unimplemented in the complete code, so removing it.

Review URL: https://codereview.chromium.org/297293007

git-svn-id: svn://svn.chromium.org/blink/trunk@175119 bbb929c8-8fbe-4397-9dbb-9b2b20218538