[Magnifier] Fix bug where magnified viewport couldn't reach edges of screen

In continuous mouse following mode, the viewport couldn't reach the edges
of the screen, and this problem increased with zoom levels. This fixes
the bug by realizing that the cursor could never reach the bottom or right
of the screen but always remained one DIP within bounds so that it could
be drawn.

Also cleans up some unused and redundant parts of
fullscreen_magnifier_controller.

could not reach bottom / right edges of the screen.

Bug: b:262037897
Test: New unittest added, would have failed without new code
Change-Id: Id7b4863daeb8c186868818576e9ada2b849b07b4
AX-Relnotes: Fixes bug where continuous mouse following mode
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4097121
Reviewed-by: Josiah Krutz <josiahk@google.com>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1082470}

• ash/accessibility/magnifier/fullscreen_magnifier_controller.cc[diff]
• ash/accessibility/magnifier/fullscreen_magnifier_controller.h[diff]
• ash/accessibility/magnifier/fullscreen_magnifier_controller_unittest.cc[diff]