FitText: Simplify BlockLayoutAlgorithm and InlineChildLayoutContext

We don't need to measure a per-block scaling factor during Layout*()
because we can do it with FragmentItems after calling Layout*(). This
CL removes some unnecessary code for the measure-during-layout approach.

This CL has no behavior changes.

Bug: 417306102
Change-Id: I045e1324a62fcc42c835b0074f2ad7cfc6490d92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6887368
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1506947}