Revert "[web] Paragraph.longestLine doesn't need to check for isSingleLine anymore (flutter#16736)"

This reverts commit 7758967.

Author: NoamDev

lib/web_ui/lib/src/engine/text/paragraph.dart

@@ -210,6 +210,12 @@ class EngineParagraph implements ui.Paragraph {
       return maxWidth;
     }
 
+    // In the single-line case, the longest line is equal to the maximum
+    // intrinsic width of the paragraph.
+    if (_measurementResult.isSingleLine) {
+      return _measurementResult.maxIntrinsicWidth;
+    }
+
     // If we don't have any line metrics information, there's no way to know the
     // longest line in a multi-line paragraph.
     return 0.0;