Reland of Extract iOS SDK helpers to separate target.

webrtc/test/BUILD.gn

Index: webrtc/test/BUILD.gn
diff --git a/webrtc/test/BUILD.gn b/webrtc/test/BUILD.gn
index c696979dd11105b7c4aed5bc81b74c28fca2978f..34500df6fa77dcdb3023fcaf38e8e27444fcff3c 100644
--- a/webrtc/test/BUILD.gn
+++ b/webrtc/test/BUILD.gn
@@ -297,8 +297,10 @@
     "testsupport/fileutils.cc",
     "testsupport/fileutils.h",
   ]
+  deps = []
   if (is_ios) {
     sources += [ "testsupport/iosfileutils.mm" ]
+    deps += [ "../sdk:rtc_sdk_helpers_objc" ]
   }
   visibility = [ ":*" ]
 }