Reset GC performance stats at zygote fork.

So GCs before a zygote fork won't be attributed to an app.

(cherry pick commit  Ib37bc587e0f039ef8faeabe63dec19de49501863)

Bug: 21491908
Change-Id: Ib37bc587e0f039ef8faeabe63dec19de49501863
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index cb7f5f3..66ec7cc 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -646,6 +646,10 @@
 
   // Create the thread pools.
   heap_->CreateThreadPool();
+  // Reset the gc performance data at zygote fork so that the GCs
+  // before fork aren't attributed to an app.
+  heap_->ResetGcPerformanceInfo();
+
   if (jit_.get() == nullptr && jit_options_->UseJIT()) {
     // Create the JIT if the flag is set and we haven't already create it (happens for run-tests).
     CreateJit();