Fix viewport mapping for OOPIF roots in GeometryMapper fast path

The GeometryMapper fast path in MapToVisualRectInAncestorSpace had an
early return when the object being mapped was the same as the target
ancestor. While correct for standard ancestor mapping, this was
incorrect when mapping a LayoutView to the viewport (ancestor ==
nullptr).

In the fast path, mapping to the viewport internalizes the ancestor as
the LayoutView itself. The early return caused the code to skip
ApplyViewportClippingAndOffsets, which is responsible for applying the
remote viewport transform in OOPIF processes. As a result, OOPIF roots
incorrectly returned process-local coordinates instead of absolute
coordinates when mapping to the viewport.

This fix ensures the early return only triggers if we are specifically
mapping to the object's own space and NOT to the viewport. This allows
remote transforms to be correctly applied to the LayoutView, making its
geometry consistent with its children and other frames.

Gated behind the new FixVisualRectRemoteViewportTransform runtime
feature as a kill-switch.

Bug: 496593975
Change-Id: I7af640b3178e3a4abc241bd609a4934f030e266c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7745904
Commit-Queue: Nan Lin <linnan@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1613267}