Bug 1303910 - remove superfluous FallibleTArray in Blob.cpp; r=baku

This array only exists to be initialized fallibly so we can swap it into
the infallible array we actually want to use.  But we can fallibly
initialize the infallible array directly, so let's do that.

dom/ipc/Blob.cpp

@@ -823,14 +823,11 @@ CreateBlobImpl(const nsTArray<BlobData>& aBlobDatas,
     return blobImpl.forget();
   }
 
-  FallibleTArray<RefPtr<BlobImpl>> fallibleBlobImpls;
-  if (NS_WARN_IF(!fallibleBlobImpls.SetLength(aBlobDatas.Length(), fallible))) {
+  nsTArray<RefPtr<BlobImpl>> blobImpls;
+  if (NS_WARN_IF(!blobImpls.SetLength(aBlobDatas.Length(), fallible))) {
     return nullptr;
   }
 
-  nsTArray<RefPtr<BlobImpl>> blobImpls;
-  fallibleBlobImpls.SwapElements(blobImpls);
-
   const bool hasRecursed = aMetadata.mHasRecursed;
   aMetadata.mHasRecursed = true;