Bug 1203680 P2 Implement an NS_InputStreamIsCloneable() method. r=froydnj

xpcom/io/nsStreamUtils.cpp

@@ -857,6 +857,17 @@ NS_FillArray(FallibleTArray<char>& aDest, nsIInputStream* aInput,
   return rv;
 }
 
+bool
+NS_InputStreamIsCloneable(nsIInputStream* aSource)
+{
+  if (!aSource) {
+    return false;
+  }
+
+  nsCOMPtr<nsICloneableInputStream> cloneable = do_QueryInterface(aSource);
+  return cloneable && cloneable->GetCloneable();
+}
+
 nsresult
 NS_CloneInputStream(nsIInputStream* aSource, nsIInputStream** aCloneOut,
                     nsIInputStream** aReplacementOut)

xpcom/io/nsStreamUtils.h

@@ -264,6 +264,12 @@ extern NS_METHOD
 NS_FillArray(FallibleTArray<char>& aDest, nsIInputStream* aInput,
              uint32_t aKeep, uint32_t* aNewBytes);
 
+/**
+ * Return true if the given stream can be directly cloned.
+ */
+extern bool
+NS_InputStreamIsCloneable(nsIInputStream* aSource);
+
 /**
  * Clone the provided source stream in the most efficient way possible.  This
  * first attempts to QI to nsICloneableInputStream to use Clone().  If that is