REGRESSION(r140024): child of fixed div inside of absolute div does not get resized when window is resized

When detecting whether a fixed pos object needed to move with an absolute ancestor we checked
whether the logical width of the fixed pos object had changed. This check prevented the fixed pos
object from detecting later on that it needed to relayout its children in layoutBlock(). So recompute the width
for our check instead of updating it.

This is a merge of https://trac.webkit.org/changeset/149640 from robert@webkit.org

BUG=344420

Review URL: https://codereview.chromium.org/171463003

git-svn-id: svn://svn.chromium.org/blink/trunk@167389 bbb929c8-8fbe-4397-9dbb-9b2b20218538