Adds a gn argument for boringssl path

runtime/bin/BUILD.gn

 # Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 
 declare_args() {
   dart_io_support = false
+  dart_boringssl_path = "../../third_party/boringssl"
 }
 
 
 resources_sources_gypi =
     exec_script("../../tools/gypi_to_gn.py",
                 [rebase_path("vmservice/vmservice_sources.gypi")],
                 "scope",
                 ["vmservice/vmservice_sources.gypi"])
 
 # Generate a resources.cc file for the service isolate without Observatory.
@@ skipping 305 matching lines @@
     ]
   }
   set_sources_assignment_filter(custom_sources_filter)
 
   if (is_mac || is_ios) {
     libs = [
       "Security.framework",
     ]
   } else {
     deps = [
-      "//third_party/boringssl",
+      rebase_path(dart_boringssl_path, "."),
     ]
   }
 
   sources = io_impl_sources_gypi.sources + builtin_impl_sources_gypi.sources
   sources += [
     "builtin_nolib.cc",
     "builtin_natives.cc",
     "io_natives.cc",
     "io_natives.h",
     "embedded_dart_io.cc",
     "embedded_dart_io.h",
     "log_android.cc",
     "log_linux.cc",
     "log_macos.cc",
     "log_win.cc",
     "log.h",
   ]
 
   include_dirs = [
     "..",
     "//third_party"
   ]
 }