Use StyleNode in StockMenu

Use StyleNode in StockMenu

StockMenu was creating a container for the sole purpose of applying style to
PopupMenu. Now we just use a StyleNode.

Also, I've reverted the change to make box-sizing default to border-box. It
turns out that CL wasn't effective because we didn't use the initialBoxSizing
function to initialize box sizing. I've made us use initialBoxSizing but switch
the default back to content-box because actually using border-box breaks a
bunch of stuff.

R=eseidel@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/7f4cd25744763f72bf7039a28ef129a363f99652

[eseidel, 2015-03-20 17:15:47]

lgtm

[abarth-chromium, 2015-03-20 17:16:30]

Committed patchset #1 (id:1) manually as
7f4cd25744763f72bf7039a28ef129a363f99652 (presubmit successful).