Fix bugs with chrome_root

PerfBenchmark has method _GetVariationsBrowserArgs, which returns
fieldtrial args, read from file 'testing/variations/fieldtrial_testing_config.json'.
Right now chrome_root is used as root directory for computing absolute path
to  'fieldtrial_testing_config.json', but the real root directory for
this file is 'src'.
The main idea is that chrome_root not obligatory pointing to 'src'.
chrome_root is actually a directory, which has built chrome binaries in
'out/Default', 'out/Debug, etc. Using chrome_root as root for
'fieldtrial_testing_config.json' causes errors, because code can not find
'fieldtrial_testing_config.json' if chrome_root is differ from 'src'.
This CL tries to use 'src' as a root directory for 'fieldtrial_testing_config.json'
explicitly.

Changes in 'scripts_smoke_unittest.py' is needed to fix bugs, caused
by specifying chrome_root, differ from 'src'.

Change-Id: Ie51d47e7ff38ac84ef909c049ae8da5fa2421229
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3695412
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Dmitry Shcherbinin <cher-di@yandex-team.ru>
Cr-Commit-Position: refs/heads/main@{#1013149}