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This CL limits the height of the Feedback dialog to the maximum
available height of the user's screen.

This CL also fixes the cancel / show buttons to the bottom of the
window so that they remain visible in the overflow state.

Also fixes a mismatch in feedback height constants.

Dialog with overflow:
https://drive.google.com/file/d/1tPJumvN2NmlSj5pI5htr18QHBJKCi6W9/view?usp=sharing

Dialog with no overflow:
https://drive.google.com/file/d/1OB1arIhF4t0cWfKZdTYGrmuk2vCWcczR/view?usp=sharing

Bug: 1065889
Change-Id: If49afa489445df770063d3b7feda889900bcfdcb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586759Reviewed-by: Miriam Zimmerman <mutexlox@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837686}