[swarming] Isolate custom libcxx.

BUG=chromium:535160
LOG=n

Review URL: https://codereview.chromium.org/1420473003

Cr-Commit-Position: refs/heads/master@{#31612}

build/isolate.gypi

@@ -70,6 +70,7 @@
         '--config-variable', 'OS=<(OS)',
         '--config-variable', 'component=<(component)',
         '--config-variable', 'target_arch=<(target_arch)',
+        '--config-variable', 'use_custom_libcxx=<(use_custom_libcxx)',
         '--config-variable', 'v8_use_external_startup_data=<(v8_use_external_startup_data)',
         '--config-variable', 'v8_use_snapshot=<(v8_use_snapshot)',
       ],

src/base.isolate

@@ -3,6 +3,13 @@
 # found in the LICENSE file.
 {
   'conditions': [
+    ['use_custom_libcxx==1', {
+      'variables': {
+        'files': [
+          '<(PRODUCT_DIR)/lib/libc++.so',
+        ],
+      },
+    }],
     ['v8_use_snapshot=="true" and v8_use_external_startup_data==1', {
       'variables': {
         'files': [