Remove MarkOnlyThis calls from layoutSpecialExcludedChild.

This is part of the larger project of getting rid of MarkOnlyThis.

Also, removed a FIXME that doesn't really make sense from
Source/core/rendering/SubtreeLayoutScope.h. We already assert that it's
only used from within layout and the MarkOnlyThis assert doesn't make
sense since MarkOnlyThis is going away entirely.

Review URL: https://chromiumcodereview.appspot.com/22867012

git-svn-id: svn://svn.chromium.org/blink/trunk@156268 bbb929c8-8fbe-4397-9dbb-9b2b20218538