Guard some extensions build rules with enable_extensions.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 95cdd60e09dc14a68b2b2d8403462e037c4d5936..9378182f8a52c9dd195a42161af22decda59324e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -102,8 +102,6 @@ group("gn_all") {
       "//chrome/test/chromedriver:chromedriver_unittests",
       "//components/sync/tools:sync_client",
       "//components/sync/tools:sync_listen_notifications",
-      "//extensions:extensions_browsertests",
-      "//extensions:extensions_unittests",
       "//gpu/gles2_conform_support:gles2_conform_test",
       "//gpu/khronos_glcts_support:khronos_glcts_test",
       "//jingle:jingle_unittests",
@@ -207,7 +205,11 @@ group("gn_all") {
   deps += root_extra_deps
 
   if (enable_extensions) {
-    deps += [ "//extensions/shell:app_shell_unittests" ]
+    deps += [
+      "//extensions:extensions_browsertests",
+      "//extensions:extensions_unittests",
+      "//extensions/shell:app_shell_unittests",
+    ]
   }
 
   if (enable_remoting) {