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>
diff --git a/scripts/dawn_features.gni b/scripts/dawn_features.gni
index d86f218..a74942b 100644
--- a/scripts/dawn_features.gni
+++ b/scripts/dawn_features.gni
@@ -33,7 +33,7 @@
 
   # Enables the compilation of Dawn's OpenGL backend
   # (best effort, non-conformant)
-  dawn_enable_opengl = is_linux
+  dawn_enable_opengl = is_linux && !is_chromeos
 
   # Enables the compilation of Dawn's Vulkan backend
   dawn_enable_vulkan = is_linux || is_win