[MemSheriff] Exclude tests prematurely added that include leaks and failures under Memcheck

TBR=tfarina@chromium.org
BUG=402257

Review URL: https://codereview.chromium.org/480823003

Cr-Commit-Position: refs/heads/master@{#290210}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290210 0039d316-1c4b-4281-b951-d872f2087c98

tools/valgrind/gtest_exclude/extensions_unittests.gtest-memcheck.txt

+# These contain un-analyzed test failures and leaks
+# http://crbug.com/402257
+ApiTestBaseTest.*
+SerialApiTest.*