Add extensions_unittests suite to Linux Clang (dbg) bot.

BUG=348066
TEST=None
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/585283002

Cr-Commit-Position: refs/heads/master@{#295902}

testing/buildbot/chromium.linux.json

@@ -190,6 +190,7 @@
       "crypto_unittests",
       "device_unittests",
       "display_unittests",
+      "extensions_unittests",
       "google_apis_unittests",
       "ipc_mojo_unittests",
       "ipc_tests",