In the css3-text spec:

The start and end edges of a line box are determined by the inline base
direction of the line box. In most cases, this is given by its
containing block's computed ‘direction’. However if its containing block
has ‘unicode-bidi: plaintext’, the inline base direction the line box must
be determined by the base direction of the bidi paragraph to which it
belongs: that is, the bidi paragraph for which the line box holds content.

This patch just implements the behavior described above modifying
updateLogicalWidthForAlignment and setting the bidi level of the
RootInlineBox.

Additionally, modify two layout tests to make it conforming to the spec.

BUG=280709

Review URL: https://chromiumcodereview.appspot.com/23567004

git-svn-id: svn://svn.chromium.org/blink/trunk@157263 bbb929c8-8fbe-4397-9dbb-9b2b20218538