[A11yPerformance][a11y] Compute on-screen nodes based on viewport

This commit introduces a new approach to determine
which nodes are considered "on-screen" for
accessibility purposes when the `kOnScreenOnly`
AXMode flag is enabled.

The new logic computes the location of each node
and classifies it as on-screen if any part of its
vertical extent intersects with the user's current
viewport. Content displaced horizontally (left or
right) is still included as long as it's within
the viewport's y-axis.

To improve the user experience by pre-serializing
potentially visible content, nodes that are
located just below the viewport are also included.

This change removes the previous method of
determining off-screen nodes.

**Performance considerations:** As this is guarded
behind an experimental flag, we will monitor its
performance during testing. If any performance
issues are identified, we will address them in
subsequent optimizations.

Test: out/Default/content_browsertests --gtest_filter="*OnScreenModeDumpAccessibilityTreeTest*/blink"
Change-Id: I789eb630cb5f2c04dd8eec506a05a4ede316c20e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6448919
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
Cr-Commit-Position: refs/heads/main@{#1446683}