Fall back to UTF-8 instead of Windows-1252 as last-effort.

If the following applies, fall back to UTF-8:
- No content encoding is known
- No encoding override is set
- There is no specific fallback for the browser locale

dom/encoding/FallbackEncoding.cpp

@@ -94,7 +94,7 @@ FallbackEncoding::Get(nsACString& aFallback)
 
   if (NS_FAILED(nsUConvPropertySearch::SearchPropertyValue(
       localesFallbacks, ArrayLength(localesFallbacks), locale, mFallback))) {
-    mFallback.AssignLiteral("windows-1252");
+    mFallback.AssignLiteral("UTF-8");
   }
 
   aFallback = mFallback;