Convert sandbox_linux_unittests to run exclusively on Swarming

testing/buildbot/chromium.linux.json

Index: testing/buildbot/chromium.linux.json
diff --git a/testing/buildbot/chromium.linux.json b/testing/buildbot/chromium.linux.json
index fd50fe196e60a61d48e1884566961aed6ff3cb76..340f50e21c58114b5a849291670d4d5cea19bf3f 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -181,6 +181,9 @@
         "test": "ppapi_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sandbox_linux_unittests"
       },
       {
@@ -679,6 +682,9 @@
         "test": "remoting_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sandbox_linux_unittests"
       },
       {
@@ -975,6 +981,9 @@
         "args": [
           "--test-launcher-print-test-stdio=always"
         ],
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sandbox_linux_unittests"
       },
       {
@@ -1255,6 +1264,9 @@
         "test": "remoting_unittests"
       },
       {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "sandbox_linux_unittests"
       },
       {