Bug 1276724 - Make two url-classifier allocations fallible. r=gcp.

This addresses two crashes in the top #75 on 47.0.1.

toolkit/components/url-classifier/HashStore.cpp

@@ -787,7 +787,9 @@ HashStore::WriteAddPrefixes(nsIOutputStream* aOut)
 {
   nsTArray<uint32_t> chunks;
   uint32_t count = mAddPrefixes.Length();
-  chunks.SetCapacity(count);
+  if (!chunks.SetCapacity(count, fallible)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
 
   for (uint32_t i = 0; i < count; i++) {
     chunks.AppendElement(mAddPrefixes[i].Chunk());