Updated MallocHooks to collect stack traces when memory is allocated.

runtime/vm/os_linux.cc

Index: runtime/vm/os_linux.cc
diff --git a/runtime/vm/os_linux.cc b/runtime/vm/os_linux.cc
index 163f436ea632aaeabb40a9ac43173aefaac27993..e73996bc7fb0dbc2651ff65f8358c22b27bbf49f 100644
--- a/runtime/vm/os_linux.cc
+++ b/runtime/vm/os_linux.cc
@@ -287,6 +287,12 @@ void OS::DebugBreak() {
 }
 
 
+uintptr_t DART_NOINLINE OS::GetProgramCounter() {
+  return reinterpret_cast<uintptr_t>(
+      __builtin_extract_return_addr(__builtin_return_address(0)));
+}
+
+
 char* OS::StrNDup(const char* s, intptr_t n) {
   return strndup(s, n);
 }