contenttest: Print stderr
Kill Servo instead of displaying about:failure when running content tests

Fixes #1380.
contenttest: Fail if Servo exits with non-zero status

Remove "let stdout" to make the borrow checker happy.
Don't clear buffers if we have no native graphics context

Fixes ./servo -z for content that calls window.close(), such as content tests.
auto merge of #1382 : kmcallister/servo/headless, r=jdm

r? @jdm
layout: If the border style is "none", treat its width as 0.

Closes #1383.
auto merge of #1384 : pcwalton/servo/border-style-none, r=kmcallister

Closes #1383.

r? @kmcallister
layout: Remove unused `content_box_width` field from `Box`.

Shaves 4 bytes off the size of `Box`.
auto merge of #1390 : pcwalton/servo/decontentboxwidth, r=metajack

Shaves 4 bytes off the size of `Box`.