sunfish: Draw gradient border when selecting a capture region.

Draw a gradient border around the capture region when the user is
selecting a capture region in sunfish mode (only the initial "selection"
phase, and not subsequent "fine tuning" phases).

Bug: b:395483324
Change-Id: Ib19c6c3eda6f96e23f703199a179148210efcb47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6247835
Commit-Queue: Michelle Chen <michellegc@google.com>
Reviewed-by: Elijah Hewer <hewer@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1418941}