Use Xvfb in isolated testing only if use_x11 is set (reland)

build/isolate.gypi

Index: build/isolate.gypi
diff --git a/build/isolate.gypi b/build/isolate.gypi
index fbb6d3cf18321f7153fae2256896ac9c867bd616..0e834be526b4e8dd0fab7eb71e34c9d70d7c128e 100644
--- a/build/isolate.gypi
+++ b/build/isolate.gypi
@@ -96,6 +96,7 @@
         '--config-variable', 'use_openssl=<(use_openssl)',
         '--config-variable', 'use_ozone=<(use_ozone)',
         '--config-variable', 'v8_use_external_startup_data=<(v8_use_external_startup_data)',
+        '--config-variable', 'use_x11=<(use_x11)',

[M-A Ruel, 2015/03/10 16:24:52]

Please keep the list sorted.

[spang, 2015/03/10 16:34:27]

Done.

       ],
       'conditions': [
         # Note: When gyp merges lists, it appends them to the old value.