Bug 1119232 - Fix a warning for the usage of the uninitialized gc member in JSRuntime's constructor; r=jandem

clang emits the following warning on this code:
warning: field 'gc' is uninitialized when used here [-Wuninitialized]

The warning is not an indication of a real bug since we're just taking the store
buffer's address, but we may as well silence it.

js/src/jspubtd.h

@@ -146,9 +146,9 @@ struct Runtime
     js::gc::StoreBuffer *gcStoreBufferPtr_;
 
   public:
-    explicit Runtime(js::gc::StoreBuffer *storeBuffer)
+    Runtime()
       : needsIncrementalBarrier_(false)
-      , gcStoreBufferPtr_(storeBuffer)
+      , gcStoreBufferPtr_(nullptr)
     {}
 
     bool needsIncrementalBarrier() const {
@@ -161,6 +161,11 @@ struct Runtime
         return reinterpret_cast<JS::shadow::Runtime*>(rt);
     }
 
+  protected:
+    void setGCStoreBufferPtr(js::gc::StoreBuffer *storeBuffer) {
+        gcStoreBufferPtr_ = storeBuffer;
+    }
+
     /* Allow inlining of PersistentRooted constructors and destructors. */
   private:
     template <typename Referent> friend class JS::PersistentRooted;

js/src/vm/Runtime.cpp

@@ -130,8 +130,7 @@ ReturnZeroSize(const void *p)
 }
 
 JSRuntime::JSRuntime(JSRuntime *parentRuntime)
-  : JS::shadow::Runtime(&gc.storeBuffer),
-    mainThread(this),
+  : mainThread(this),
     parentRuntime(parentRuntime),
     interrupt_(false),
     interruptPar_(false),
@@ -230,6 +229,8 @@ JSRuntime::JSRuntime(JSRuntime *parentRuntime)
     oomCallback(nullptr),
     debuggerMallocSizeOf(ReturnZeroSize)
 {
+    setGCStoreBufferPtr(&gc.storeBuffer);
+
     liveRuntimesCount++;
 
     /* Initialize infallibly first, so we can goto bad and JS_DestroyRuntime. */