[iOS/GN] Fix the compilation of "all" on iOS with gn.

services/shell/standalone/BUILD.gn

Index: services/shell/standalone/BUILD.gn
diff --git a/services/shell/standalone/BUILD.gn b/services/shell/standalone/BUILD.gn
index 9eb1c021a5ff2e7b32a9fb64b520382cd3a64f2e..74adfb5635770532b3c735b82f16b1bc3e4bf5f5 100644
--- a/services/shell/standalone/BUILD.gn
+++ b/services/shell/standalone/BUILD.gn
@@ -21,8 +21,6 @@ executable("standalone") {
 
 source_set("lib") {
   sources = [
-    "context.cc",
-    "context.h",
     "desktop/launcher_process.cc",
     "desktop/launcher_process.h",
     "desktop/main_helper.cc",
@@ -50,6 +48,13 @@ source_set("lib") {
     "//services/tracing",
   ]
 
+  if (!is_ios) {
+    sources += [
+      "context.cc",
+      "context.h",
+    ]
+  }
+
   # This target includes some files behind #ifdef OS... guards. Since gn is not
   # smart enough to understand preprocess includes, it does complains about
   # these includes when not using the build files for that OS. Suppress checking