Use Xvfb in isolated testing only if use_x11 is set

We don't need a virtual X server for ozone testing, and it is causing
problems running the tests with swarming. This changes all isolate files
to only run Xvfb if use_x11==1 is set in GYP_DEFINES, and merges ozone
with Windows & Mac (none of which need to run their own display server).

BUG=440882
TEST=isolate.py run -s out_ozone/Debug/<various>
TBR=maruel@chromium.org

Review URL: https://codereview.chromium.org/813363003

Cr-Original-Commit-Position: refs/heads/master@{#310434}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8a7be0714892c0c6360c45d0d602c119873b468e

android/pylib/utils/isolator.py

@@ -37,6 +37,7 @@ def DefaultConfigVariables():
     'target_arch': 'arm',
     'use_openssl': '0',
     'use_ozone': '0',
+    'use_x11': '0',
     'v8_use_external_startup_data': '0',
   }
 

isolate.gypi

@@ -91,6 +91,7 @@
         '--config-variable', 'use_openssl=<(use_openssl)',
         '--config-variable', 'target_arch=<(target_arch)',
         '--config-variable', 'use_ozone=<(use_ozone)',
+        '--config-variable', 'use_x11=<(use_x11)',
         '--config-variable', 'disable_nacl=<(disable_nacl)',
       ],
       'conditions': [