Vulkan: Fallback to XCB for Xlib surfaces

Chromium builds the Vulkan loader without support Xlib (because it
prefers XCB) which caused Xlib wgpu::SwapChain creation to fail on the
Vulkan backend.

This CL adds a fallback to use VK_KHR_xcb_surface if VK_KHR_xlib_surface
isn't present.

Bug: dawn:662
Change-Id: I0e0128ee6b5c75da03998dbae231d17e48bacc81
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/41180
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>

src/common/vulkan_platform.h

@@ -162,6 +162,7 @@ namespace dawn_native { namespace vulkan {
 
 #if defined(DAWN_USE_X11)
 #    define VK_USE_PLATFORM_XLIB_KHR
+#    define VK_USE_PLATFORM_XCB_KHR
 #    include "common/xlib_with_undefs.h"
 #endif  // defined(DAWN_USE_X11)
 

src/common/xlib_with_undefs.h

@@ -24,6 +24,7 @@
 // This header includes <X11/Xlib.h> but removes all the extra defines that conflict with
 // identifiers in internal code. It should never be included in something that is part of the public
 // interface.
+#include <X11/Xlib-xcb.h>
 #include <X11/Xlib.h>
 
 #undef Success