Bug 1362194 - part 1 - add a fallible CopyASCIItoUTF16 function; r=mccr8

We already have all the machinery to expose a function like this, we
just need to write it.

xpcom/string/nsReadableUtils.cpp

@@ -87,9 +87,20 @@ LossyCopyUTF16toASCII(const nsAString& aSource, nsACString& aDest)
 
 void
 CopyASCIItoUTF16(const nsACString& aSource, nsAString& aDest)
+{
+  if (!CopyASCIItoUTF16(aSource, aDest, mozilla::fallible)) {
+    // Note that this may wildly underestimate the allocation that failed, as
+    // we report the length of aSource as UTF-16 instead of UTF-8.
+    aDest.AllocFailed(aDest.Length() + aSource.Length());
+  }
+}
+
+bool
+CopyASCIItoUTF16(const nsACString& aSource, nsAString& aDest,
+                 const mozilla::fallible_t& aFallible)
 {
   aDest.Truncate();
-  AppendASCIItoUTF16(aSource, aDest);
+  return AppendASCIItoUTF16(aSource, aDest, aFallible);
 }
 
 void

xpcom/string/nsReadableUtils.h

@@ -35,6 +35,8 @@ Distance(const nsReadingIterator<char>& aStart,
 
 void LossyCopyUTF16toASCII(const nsAString& aSource, nsACString& aDest);
 void CopyASCIItoUTF16(const nsACString& aSource, nsAString& aDest);
+MOZ_MUST_USE bool CopyASCIItoUTF16(const nsACString& aSource, nsAString& aDest,
+                                   const mozilla::fallible_t&);
 
 void LossyCopyUTF16toASCII(const char16ptr_t aSource, nsACString& aDest);
 void CopyASCIItoUTF16(const char* aSource, nsAString& aDest);