Add cast_unittests to Linux ASan LSan Tests (2).

BUG=400728
TBR=phajdan.jr@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/520543003

Cr-Commit-Position: refs/heads/master@{#292607}

testing/buildbot/chromium.memory.json

@@ -16,6 +16,7 @@
   "Linux ASan LSan Tests (2)": {
     "gtest_tests": [
       "accessibility_unittests",
+      "cast_unittests",
       "content_unittests",
       "display_unittests",
       "gfx_unittests",