Suppress keepalive thread leak in DM.

BUG=skia:

Review URL: https://codereview.chromium.org/1006143002
diff --git a/tools/valgrind.supp b/tools/valgrind.supp
index d919c86..28429c0 100644
--- a/tools/valgrind.supp
+++ b/tools/valgrind.supp
@@ -1,5 +1,17 @@
 # Pass this file to Valgrind with "--suppressions=tools/valgrind.supp"
 
+# Intentional thread / memory leak in DM.
+{
+   dm_keepalive_thread_leak
+   Memcheck:Leak
+   match-leak-kinds: possible
+   ...
+   fun:_ZN8SkThreadC1EPFvPvES0_
+   fun:_ZL15start_keepalivev
+   fun:_Z7dm_mainv
+   fun:main
+}
+
 # Third party lib, driver issues.
 {
     ati_driver_bug_1