Disable Vulkan build when is_starboard is defined

Vulkan is a new rendering (and presentation, and more) API
intended to replace E/GL(ES). Starboard doesn't define it
and it's likely unsupported by partners (and even if it is,
the status would be unknown and by experience not ready).
This CL/PR disables building it when starboard is defined.

On dependencies, //gpu is a low level folder (like //base)
so it can depend on //starboard concepts. Modifying it is
not fully desirable (complicates future uprevs), however
the CL tries to make the delta easy to rebase.

b/371272304

gpu/vulkan/features.gni

@@ -10,6 +10,7 @@ declare_args() {
   # Enable experimental vulkan backend.
   enable_vulkan =
       is_linux || is_chromeos || is_android || is_fuchsia || is_win || is_mac
+      && !use_starboard
 
   # Enable swiftshader vulkan. Disabling it can save build time, however
   # --use-vulkan=swiftshader and some tests which use swiftshader vulkan will