Add chrome test support target to GN build.

chrome/common/BUILD.gn

Index: chrome/common/BUILD.gn
diff --git a/chrome/common/BUILD.gn b/chrome/common/BUILD.gn
index 8d6e309eab6be24a5a2f3196f2c270de6fd7fc52..ed83bad437cfaa272b12b2fd56f21952e0408eaf 100644
--- a/chrome/common/BUILD.gn
+++ b/chrome/common/BUILD.gn
@@ -291,3 +291,29 @@ static_library("constants") {
     ]
   }
 }
+
+source_set("test_support") {
+  testonly = true
+  visibility = [ "//chrome/test:test_support" ]
+
+  sources = [
+    "extensions/extension_test_util.cc",
+    "extensions/extension_test_util.h",
+  ]
+
+  deps = [
+    ":common",
+    "//base",
+    "//testing/gtest",
+  ]
+
+  if (is_win || is_mac) {
+    sources += [
+      "media_galleries/picasa_test_util.cc",
+      "media_galleries/picasa_test_util.cc",
+      "media_galleries/pmp_test_util.cc",
+      "media_galleries/pmp_test_util.h",
+    ]
+  }
+
+}