Add tests for Headless Linux bot

testing/buildbot/chromium.fyi.json

 {
   "Android Asan Builder Tests (dbg)": {
     "gtest_tests": [
       {
         "args": [
           "--tool=asan"
         ],
         "test": "android_webview_unittests"
       },
       {
@@ skipping 6841 matching lines @@
         "test": "webkit_unit_tests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wtf_unittests"
       }
     ]
   },
+  "Headless Linux (dbg)": {
+    "additional_compile_targets": [
+      "headless_libs",
+      "headless_tests"
+    ],
+    "gtest_tests": [
+      {
+        "test": "headless_browsertests"
+      },
+      {
+        "test": "headless_unittests"
+      }
+    ]
+  },
   "Linux ARM": {
     "gtest_tests": [
       {
         "args": [
           "--gtest_filter=*NaCl*.*"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "browser_tests"
@@ skipping 1498 matching lines @@
       },
       {
         "args": [
           "--isolate-extensions"
         ],
         "test": "unit_tests"
       }
     ]
   }
 }