cros: Fix desks rename textfield elision.

Currently the desk rename textfield does not account for its padding
when eliding its text. This results in the first couple of letters not
being visible when a long desk name is typed.

This CL fixes this bug by making the desk rename textfield account for
its padding.

Test: manual
Bug: 1142104
Change-Id: I515e9822bd8d1321d62bb976bdfa59ad5fcb48ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505678
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Jeremy Chinsen <chinsenj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822712}