gn: Don't depend on osmesa.

third_party/mesa/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 config("mesa_headers_config") {
   include_dirs = [ "src/include" ]
   if (!use_x11) {
     defines = [ "MESA_EGL_NO_X11_HEADERS" ]
   }
 }
 
+assert(!is_ios, "mesa is not used on iOS, don't pull it in accidentally")
+
 # This directory contains checked-in files generated from the Mesa build.
 generated_src_dir = "src/chromium_gensrc"
 
 source_set("mesa_headers") {
   public_configs = [ ":mesa_headers_config" ]
 }
 
 # This config must generally be prepended to the configs list so that the Mesa
 # dirs appear before the system ones on Windows. This is necessary so that
 # #include "GL/gl.h" means to include Mesa's, not the system's.
@@ skipping 757 matching lines @@
     deps = [
       "//third_party/wayland:wayland_util",
     ]
 
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
 
     public_configs = [ ":wayland_drm_protocol_config" ]
   }
 }