Remove redundant layoutGutter() call from setFrameSize()

During scrolling, layoutGutter() was being called twice per frame:
1. From setFrameSize() (called via sizeToFit() in willLayout)
2. From textViewportLayoutControllerDidLayout()

The second call in the viewport delegate is the authoritative one that
runs after all fragment views are configured, making the first call
redundant. This reduces gutter layout work by ~50% during scroll.