layout: Make `box-sizing` only affect explicitly-specified or percentage
sizes, not automatically computed ones.

Improves GitHub significantly.
Disable html/semantics/forms/the-input-element/type-change-state.html.ini
auto merge of #3656 : Manishearth/servo/disable-type-change, r=Ms2ger

I haven't yet gotten to the root of this, but it's breaking the other PRs.
auto merge of #3615 : pcwalton/servo/box-sizing-fix, r=glennw

Improves GitHub significantly.

r? @glennw
Include leading margin in inline block size

According to the documentation for Fragment::position, the inline axis
should include margin size, so we include it for blocks. Also fix
place_float which assumed that it was not included and
assign_inline_sizes which overrode the size set in
set_inline_size_constraint_solutions.

Typically this issue was hidden by large tile sizes, but fitted tiles
makes it more common.