Make RemoveFormat command to work with mixed editable

This patch changes assertion in |ApplyStyleCommand::removeInlineStyle()| to
consider |PositionIsBeforeChildren| as same as |PositionIsBeforeAnchor| and
|offsetInContainerNode <= 0|.

In the attached test case, |firstPositionInOrBeforeNode(next.get())| returns
|PositionIsBeforeChildren| for B element. When we don't have uneditable
element, inline styles are removed by |removeInlineStyleFromElement| for
BODY element rather than each child in BODY element. Blink doesn't go the path
contains the assertion.

BUG=388212
TEST=LayoutTests/editing/execCommand/remove-foramt-mixed-editable-crash.html

Review URL: https://codereview.chromium.org/332613003

git-svn-id: svn://svn.chromium.org/blink/trunk@177448 bbb929c8-8fbe-4397-9dbb-9b2b20218538