Add extensions_unittests suite to more memory bots.

BUG=348066
TEST=None
R=bruening@chromium.org,dpranke@chromium.org,jamescook@chromium.org

Review URL: https://codereview.chromium.org/588903002

Cr-Commit-Position: refs/heads/master@{#296461}

testing/buildbot/chromium_memory_trybot.json

@@ -45,6 +45,11 @@
       "platforms": ["linux"],
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     },
+    {
+      "test": "extensions_unittests",
+      "platforms": ["linux", "win", "mac"],
+      "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
+    },
     {
       "test": "gcm_unit_tests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]