Floating legend should be added to the fieldset's anonymous block.

If fieldset's legend is floating it should be added to the existing fieldset\'s anonymous block instead of creating a nested one.
This is necessary because flexbox forces the floating legend to be non-floating and therefore if the legend needs to be floating it should be added to the fieldset's anonymous block.

BUG=661230, 670837
TEST=fast/forms/fieldset/fieldset-legend-float.html

Tested with real world websites:
https://jsbin.com/zutekilusi/1/edit?html,css,output
http://www.timico.co.uk/timico-ltd-application-form
http://codepen.io/michaelpumo/pen/VmZqrr
https://en.wikipedia.org/wiki/Special:ActiveUsers
https://jsfiddle.net/tdbthtra/

Review-Url: https://codereview.chromium.org/2550963002
Cr-Commit-Position: refs/heads/master@{#436187}
3 files changed