Add fieldtrial testing config entry for RetainOmniboxOnFocus experiment.

This CL adds a fieldtrial testing config entry for the
RetainOmniboxOnFocus experiment. This is required in order to initiate
a finch experiment on channels >= dev.

Note that this CL also updates test failures which it introduces.

Design doc: http://shortn/_uCcl1jb7oA

Bug: 347632178
Change-Id: I722497c1858b8aee5768ce646fd53b3cb1c76ab5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5902119
Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/main@{#1362762}