Run extensions_unittests under MemorySanitizer.

BUG=402257
TBR=phajdan.jr@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/520693002

Cr-Commit-Position: refs/heads/master@{#292626}

testing/buildbot/chromium.fyi.json

@@ -238,6 +238,7 @@
       "content_unittests",
       "crypto_unittests",
       "device_unittests",
+      "extensions_unittests",
       "gpu_unittests",
       "interactive_ui_tests",
       "ipc_tests",