Add missing initializtaion of last_gc_time_ns_.

Fixes some valgrind warnings.

Bug: 15426766
Change-Id: Ibaa78465739bf6919509cf87c72b25c84ed1d12c
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index 26f87ca..d7b673e 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -150,6 +150,7 @@
       verify_pre_gc_rosalloc_(verify_pre_gc_rosalloc),
       verify_pre_sweeping_rosalloc_(verify_pre_sweeping_rosalloc),
       verify_post_gc_rosalloc_(verify_post_gc_rosalloc),
+      last_gc_time_ns_(NanoTime()),
       allocation_rate_(0),
       /* For GC a lot mode, we limit the allocations stacks to be kGcAlotInterval allocations. This
        * causes a lot of GC since we do a GC for alloc whenever the stack is full. When heap