Show magnifier when dragging TouchSelectionControllerImpl handle.

The ChromeOS touch selection magnifier is one of the features being
added as part of a ChromeOS touch text editing redesign. See WIP
design doc: go/cros-touch-text-editing (mainly the Magnifier section).
The idea is that the magnifier will make it easier to see the cursor
and text, especially when the user's finger might be partly covering
the cursor/text.

From TouchSelectionControllerImpl, we show the magnifier whenever there
is an active dragging handle. The magnifier zooms in on the selection
bound corresponding to the handle being dragged.

Demo:
https://drive.google.com/file/d/1Ksas_O5jbUma1cpwI6D8jssuPNZFD06j/view?usp=sharing

Bug: b:244116654, b:264824466
Change-Id: I22ec6d188b1263bb1119c48e3dcae0d8fe82316c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4405622
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Commit-Queue: Michelle Chen <michellegc@google.com>
Cr-Commit-Position: refs/heads/main@{#1141896}