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());

toolkit/components/url-classifier/LookupCache.cpp

@@ -564,7 +564,9 @@ LookupCache::ConstructPrefixSet(AddPrefixArray& aAddPrefixes)
   Telemetry::AutoTimer<Telemetry::URLCLASSIFIER_PS_CONSTRUCT_TIME> timer;
 
   nsTArray<uint32_t> array;
-  array.SetCapacity(aAddPrefixes.Length());
+  if (!array.SetCapacity(aAddPrefixes.Length(), fallible)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
 
   for (uint32_t i = 0; i < aAddPrefixes.Length(); i++) {
     array.AppendElement(aAddPrefixes[i].PrefixHash().ToUint32());