Add tests for Headless Linux bot

testing/buildbot/chromium.fyi.json

 {
   "Android Cloud Tests": {
     "gtest_tests": [
       {
         "test": "android_webview_unittests"
       },
       {
         "test": "base_unittests"
       },
       {
@@ skipping 6186 matching lines @@
         "test": "webkit_unit_tests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wtf_unittests"
       }
     ]
   },
+  "Headless Linux": {
+    "gtest_tests": [
+      {
+        "test": "headless_browsertests"
+      }
+    ]
+  },
   "Linux ARM": {
     "gtest_tests": [
       {
         "args": [
           "--gtest_filter=*NaCl*.*"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "browser_tests"
@@ skipping 1480 matching lines @@
       },
       {
         "args": [
           "--isolate-extensions"
         ],
         "test": "unit_tests"
       }
     ]
   }
 }