Add GN isolate support for a bunch of unittests.

sandbox/linux/BUILD.gn

Index: sandbox/linux/BUILD.gn
diff --git a/sandbox/linux/BUILD.gn b/sandbox/linux/BUILD.gn
index a1a77204f5f370a1a0c65c19a32b084e7e541b28..15dedd3cb7278730af38f00fde98a9c55c1aeffa 100644
--- a/sandbox/linux/BUILD.gn
+++ b/sandbox/linux/BUILD.gn
@@ -146,6 +146,15 @@ source_set("sandbox_linux_unittests_sources") {
   }
 }
 
+# TODO(GYP): Delete this after we've converted everything to GN.
+# The _run targets exist only for compatibility w/ GYP.
+group("sandbox_linux_unittests_run") {
+  testonly = true
+  deps = [
+    ":sandbox_linux_unittests",
+  ]
+}
+
 # The main sandboxing test target.
 test("sandbox_linux_unittests") {
   deps = [