Remove spurious layouts for inline children with relative widths

RenderBlockFlow::layoutInlineChildren explicitly marked all inline
children with relative width for layout on every pass. These children
are only actually effected when the block flow's width changes, in
which case we already force layout of our children by setting
relayoutChildren to true.

It turns out there was nearly identical logic in layoutBlockChildren,
except it didn't make the mistake of forcing layout of relative width
children. Changing the code to re-use the logic.

This bug was caught while analyzing Microsoft's Holiday Lites
performance benchmark:
http://ie.microsoft.com/testdrive/Performance/HolidayLites/

BUG=333005

Review URL: https://codereview.chromium.org/132523004

git-svn-id: svn://svn.chromium.org/blink/trunk@164816 bbb929c8-8fbe-4397-9dbb-9b2b20218538