Add testing/buildbot/tryserver.blink for the GN bots.

TBR=phajdan.jr@chromium.org
BUG=454413

Review URL: https://codereview.chromium.org/1062823006

Cr-Commit-Position: refs/heads/master@{#325532}

testing/buildbot/tryserver.blink

+{
+  "android_chromium_gn_compile_rel": {
+    "additional_compile_targets": [
+      "chrome_shell_apk"
+    ],
+    "gtest_tests": []
+  },
+  "linux_chromium_gn_rel": {
+    "additional_compile_targets": [
+      "all"
+    ],
+    "gtest_tests": [
+      {
+        "test": "base_unittests"
+      }
+    ]
+  }
+}