Make LAZY_INSTANCE_INITIALIZER -Wmissing-field-initializers friendly

If libbase is compiled with -Wmissing-field-initializers this is causing
warning to be generated.

Change-Id: I446160d4c94bb59dd23f2f151004a8bfaeae832d
Reviewed-on: https://chromium-review.googlesource.com/1161927Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Luis Hector Chavez <lhchavez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580794}

base/lazy_instance.h

@@ -55,7 +55,7 @@
 
 // LazyInstance uses its own struct initializer-list style static
 // initialization, which does not require a constructor.
-#define LAZY_INSTANCE_INITIALIZER {0}
+#define LAZY_INSTANCE_INITIALIZER {}
 
 namespace base {