[swarming] Remove hard-coded shared library configs

gni/isolate.gni

 # Copyright 2016 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/sanitizers/sanitizers.gni")
 import("//third_party/icu/config.gni")
 import("v8.gni")
 
 declare_args() {
   # Sets the test isolation mode (noop|prepare|check).
@@ skipping 48 matching lines @@
       if (is_tsan) {
         tsan = "1"
       } else {
         tsan = "0"
       }
       if (is_cfi) {
         cfi_vptr = "1"
       } else {
         cfi_vptr = "0"
       }
-      if (use_custom_libcxx) {
-        custom_libcxx = "1"
-      } else {
-        custom_libcxx = "0"
-      }
       if (target_cpu == "x86") {
         target_arch = "ia32"
       } else {
         target_arch = target_cpu
       }
       if (is_debug) {
         configuration_name = "Debug"
       } else {
         configuration_name = "Release"
       }
@@ skipping 68 matching lines @@
         "tsan=$tsan",
         "--config-variable",
         "coverage=0",
         "--config-variable",
         "sanitizer_coverage=0",
         "--config-variable",
         "component=$component",
         "--config-variable",
         "target_arch=$target_arch",
         "--config-variable",
-        "use_custom_libcxx=$custom_libcxx",
-        "--config-variable",
         "v8_use_external_startup_data=$use_external_startup_data",
         "--config-variable",
         "v8_use_snapshot=$use_snapshot",
       ]
 
       if (is_win) {
         args += [
           "--config-variable",
           "msvs_version=2013",
         ]
       } else {
         args += [
           "--config-variable",
           "msvs_version=0",
         ]
       }
     }
   }
 }