testing/buildbot: Run capture_unittests in single process on ChromeOS

The current ChromeOS's camera stack (cros-camera) assumes that
capture_unittests would run in a single process.

Bug: 881682
Change-Id: I7a89f63ee16c7eb97b83b15ed46ee8061d1d601d
Reviewed-on: https://chromium-review.googlesource.com/1212433
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589621}

testing/buildbot/chromium.chromiumos.json

@@ -49,6 +49,9 @@
         "test": "cacheinvalidation_unittests"
       },
       {
+        "args": [
+          "--test-launcher-jobs=1"
+        ],
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [

testing/buildbot/test_suites.pyl

@@ -370,7 +370,11 @@
     },
     'base_unittests': {},
     'cacheinvalidation_unittests': {},
-    'capture_unittests': {},
+    'capture_unittests': {
+      'args': [
+        '--test-launcher-jobs=1',
+      ],
+    },
     'cc_unittests': {},
     'chromeos_unittests': {
       'args': [