Bug 1362449 - part 3 - templatify core nsCString base64 encode routine; r=erahm

The nsACString -> nsACString encode routine has several checks in it for
correct operation, and the nsAString -> nsAString encode routine relies
on those checks happening via the nsACString -> nsACString routine.
Once we start encoding nsAStrings directly, we'll still need those
checks, and the easiest way to ensure they happen is to move the core
base64 encode logic for strings into a templated helper.

xpcom/io/Base64.cpp

@@ -326,8 +326,9 @@ Base64Encode(const char* aBinary, uint32_t aBinaryLen, char** aBase64)
   return NS_OK;
 }
 
-nsresult
-Base64Encode(const nsACString& aBinary, nsACString& aBase64)
+template<typename T>
+static nsresult
+Base64EncodeHelper(const T& aBinary, T& aBase64)
 {
   // Check for overflow.
   if (aBinary.Length() > (UINT32_MAX / 4) * 3) {
@@ -346,7 +347,7 @@ Base64Encode(const nsACString& aBinary, nsACString& aBase64)
     return NS_ERROR_OUT_OF_MEMORY;
   }
 
-  char* base64 = aBase64.BeginWriting();
+  typename T::char_type* base64 = aBase64.BeginWriting();
   Encode(aBinary.BeginReading(), aBinary.Length(), base64);
   base64[base64Len] = '\0';
 
@@ -354,6 +355,12 @@ Base64Encode(const nsACString& aBinary, nsACString& aBase64)
   return NS_OK;
 }
 
+nsresult
+Base64Encode(const nsACString& aBinary, nsACString& aBase64)
+{
+  return Base64EncodeHelper(aBinary, aBase64);
+}
+
 nsresult
 Base64Encode(const nsAString& aBinary, nsAString& aBase64)
 {