Fix border parsing to work again

I accidentally broke border parsing when
removing CSS color keyword hacks.

While debugging this I also ran into a
race condition in the debugger, which
is fixed here.

Add --gdb argument to skydb

Makes it trivial to drop into gdb having launched
mojo_shell/sky with the right arguments.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/673073002