Runtime::PreZygoteFork returns void, not boolean.

This method aborts on failure (as it should) and unconditionally
returns true, so making it return void simplifies callers.

Change-Id: Iae39bd327f20311579ece47efa8afd1be7defce9
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index a830018..9d2cb96 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -269,9 +269,8 @@
   // notreached
 }
 
-bool Runtime::PreZygoteFork() {
+void Runtime::PreZygoteFork() {
   heap_->PreZygoteFork();
-  return true;
 }
 
 void Runtime::CallExitHook(jint status) {