Crashes with GTK 3.16

[tdaede]

(miniservo:26673): Gtk-WARNING **: gtk_widget_size_allocate(): attempt to allocate widget with width -7 and height 1
Segmentation fault (core dumped)

The full gdb backtrace is here:

#0  0x000004d8ec814853 in  ()
#1  0x00000000004fe972 in CefCToCpp::UnderlyingAddRef() const ()
#2  0x00000000004fe896 in CefCToCpp::AddRef() const ()
#3  0x00000000004ebcea in scoped_refptr::scoped_refptr(CefFrame*) ()
#4  0x00000000004eb96b in CefRefPtr::CefRefPtr(CefFrame*) ()
#5  0x00000000004eb9d1 in CefCToCpp::Wrap(_cef_frame_t*) ()
#6  0x00000000005356df in load_handler_on_load_start(_cef_load_handler_t*, _cef_browser_t*, _cef_frame_t*) ()
#7  0x00007ffff1897e27 in embedding::interfaces::cef_life_span_handler::CefLifeSpanHandler::on_after_created (self=0x7fffffffd458, browser=...)
    at interfaces/cef_life_span_handler.rs:320
#8  0x00007ffff18967d9 in embedding::browser::browser_callback_after_created (browser=...) at browser.rs:268
        client = {c_object = 0x12c29e8}
        life_span_handler = {c_object = 0x12fc2a8}
#9  0x00007ffff18982ff in embedding::browser::browser_host_create (window_info=0x7fffffffd9b8, client=..., url=0xc2ff20, callback_executed=true)
    at browser.rs:286
        browser = {c_object = 0x131ec70}
#10 0x00007ffff189c2e1 in embedding::browser::cef_browser_host_create_browser_sync (window_info=0x7fffffffd9b8, client=0x12c29e8, url=0xc2ff20, _browser_settings=0x7fffffffd888, _request_context=0x0) at browser.rs:319
        _request_context = {c_object = 0x0}
        _browser_settings = 0x7fffffffd888
        client = {c_object = 0x1d1d1d1d1d1d1d1d}
        _request_context = {c_object = 0x1d1d1d1d1d1d1d1d}
        _browser_settings = 0x7fffffffd888
        url = 0xc2ff20
        client = {c_object = 0x1d1d1d1d1d1d1d1d}
        window_info = 0x7fffffffd9b8
#11 0x00007ffff189c15a in cef_browser_host_create_browser_sync ()
    at /home/thomas/sandbox/miniservo-gtk/../servo/ports/cef/target/debug/libembedding-84bdec697f0cb265.so
#12 0x00000000004ec99b in CefBrowserHost::CreateBrowserSync(CefWindowInfo const&, CefRefPtr, CefStringBase const&, CefStructBase const&, CefRefPtr) ()
#13 0x00000000004af0c3 in ServoTab::init(TabEventHandler*, _GtkNotebook*) ()
#14 0x00000000004b0b49 in MiniServo::add_tab() ()
#15 0x00000000004b0a2b in MiniServo::MiniServo(int, char**) ()
#16 0x00000000004ad8a1 in main ()