Add missing initialization for low survival rate period.

R=verwaest@chromium.org
BUG=v8:2403

Review URL: https://codereview.chromium.org/11419002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12964 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap.cc

@@ -137,6 +137,7 @@ Heap::Heap()
       tracer_(NULL),
       young_survivors_after_last_gc_(0),
       high_survival_rate_period_length_(0),
+      low_survival_rate_period_length_(0),
       survival_rate_(0),
       previous_survival_rate_trend_(Heap::STABLE),
       survival_rate_trend_(Heap::STABLE),