Temporarily excluding failing test on valgrind/drmemory bots

BUG=427729
TBR=lanwei@chromium.org

Review URL: https://codereview.chromium.org/680183003

Cr-Commit-Position: refs/heads/master@{#301494}

tools/valgrind/gtest_exclude/aura_unittests.gtest.txt

 # Flaky under Valgrind, see http://crbug.com/348331
 WindowEventDispatcherTest.TouchMovesHeld
+
+# Failing on cros/win. crbug.com/427729
+GestureRecognizerTest.GestureEventSmallPinchEnabled