Add gn_unittests to linux bots.

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 48ed3244c5fcb6756446602aa6f3e282d3019624..ed29387268c22db523d7f2678dab5a9275f69ed8 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -1,6 +1,7 @@
 {
   "Linux Tests": {
     "gtest_tests": [
+      "gn_unittests",
       {
         "test": "accessibility_unittests",
         "swarming": {
@@ -195,6 +196,7 @@
   },
   "Linux Tests (dbg)(1)(32)": {
     "gtest_tests": [
+      "gn_unittests",
       {
         "test": "accessibility_unittests",
         "swarming": {
@@ -363,6 +365,7 @@
   },
   "Linux Tests (dbg)(1)": {
     "gtest_tests": [
+      "gn_unittests",
       {
         "test": "accessibility_unittests",
         "swarming": {
@@ -543,6 +546,7 @@
   },
   "Linux Clang (dbg)": {
     "gtest_tests": [
+      "gn_unittests",
       "accessibility_unittests",
       "app_shell_unittests",
       "base_unittests",