Bug 1366218 - Use fast path to check empty value. r=smaug

HTMLInputElement::IsValueEmpty uses DocumentEncoder when input element has focus.  DocumentEncoder is slow, so we should use fast path (nsTextEditorState::HasNonEmptyValue) to check whether value is empty.

MozReview-Commit-ID: 1JxAeZ6z5A4

dom/html/HTMLInputElement.cpp

@@ -1788,6 +1788,10 @@ HTMLInputElement::GetNonFileValueInternal(nsAString& aValue) const
 bool
 HTMLInputElement::IsValueEmpty() const
 {
+  if (GetValueMode() == VALUE_MODE_VALUE && IsSingleLineTextControl(false)) {
+    return !mInputData.mState->HasNonEmptyValue();
+  }
+
   nsAutoString value;
   GetNonFileValueInternal(value);