Bug 1132499 - part 1 - convert nsTextEditorState::mValue to use Maybe instead of heap allocation; r=ehsan

This change is just a minor tidying; we need to distinguish between
"have a value" and "don't have a value" in nsTextEditorState::GetValue,
but we can do better than heap-allocating the string.

dom/html/nsTextEditorState.cpp

@@ -2139,7 +2139,7 @@ nsTextEditorState::SetValue(const nsAString& aValue, uint32_t aFlags)
     }
   } else {
     if (!mValue) {
-      mValue = new nsCString;
+      mValue.emplace();
     }
     nsString value;
     if (!value.Assign(newValue, fallible)) {