Update BUILD flags to support Chrome OS

This CL updates a few flags to support Chrome OS:

- Disables the OpenGL backend on Chrome OS builds.
- Removes the X11 dependency for Chrome OS builds because we do not
support X11.
- Removes the XCB dependency (again, no X11).

BUG=chromium:993457
TEST=e2e and unit tests build and pass on Intel Chrome OS devices

Change-Id: I1cb06453ccc94d1b68a6998ea635bddd6fb7b5ad
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/10100
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

third_party/BUILD.gn

@@ -122,7 +122,7 @@ config("vulkan_headers_config") {
   if (is_win) {
     defines = [ "VK_USE_PLATFORM_WIN32_KHR" ]
   }
-  if (is_linux) {
+  if (is_linux && !is_chromeos) {
     defines = [ "VK_USE_PLATFORM_XCB_KHR" ]
   }
   if (is_android) {