Handle crash when submitting an injected form on an "image page".

Source/core/platform/network/FormDataBuilder.cpp

Index: Source/core/platform/network/FormDataBuilder.cpp
diff --git a/Source/core/platform/network/FormDataBuilder.cpp b/Source/core/platform/network/FormDataBuilder.cpp
index 117a5beb7a275bd061696a4ba1eadb4453a7dc5c..5321f3b4d7b15f3894d5f8d5ef5ceac180c7ee02 100644
--- a/Source/core/platform/network/FormDataBuilder.cpp
+++ b/Source/core/platform/network/FormDataBuilder.cpp
@@ -74,7 +74,7 @@ static void appendQuotedString(Vector<char>& buffer, const CString& string)
     }
 }
 
-WTF::TextEncoding FormDataBuilder::encodingFromAcceptCharset(const String& acceptCharset, const String& inputEncoding)
+WTF::TextEncoding FormDataBuilder::encodingFromAcceptCharset(const String& acceptCharset, const String& inputEncoding, const String& defaultCharset)
 {
     String normalizedAcceptCharset = acceptCharset;
     normalizedAcceptCharset.replace(',', ' ');
@@ -90,6 +90,13 @@ WTF::TextEncoding FormDataBuilder::encodingFromAcceptCharset(const String& accep
             return encoding;
     }
 
+    if (inputEncoding.isEmpty()) {
+        if (defaultCharset.isEmpty())
+            return WTF::UTF8Encoding();
+
+        return defaultCharset;
+    }
+
     return inputEncoding;
 }