[UI] Textarea cursor should respect textarea viewport

Before this CL:
Upon mouse wheel changes, the cursor is moved and keeps blinking even
if it is outside the viewport of the text area.
(check the attached bug for a video)

After this CL:
The visibility of the cursor respects the bounds of the textarea viewport

Bug: 1434319
Change-Id: Ie1de16a9942761b3318199d61098c0b129809c4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4440670
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1132174}