Fix RebaselineTest._copy_existing_baseline() to not assume that fallback directory names match port names
Fix RebaselineTest._copy_existing_baseline() to not assume that fallback directory names match port names

Port.baseline_version_dir() provides the default baseline path and this is not
guaranteed to match the port name. So instead of making this assumption, search
for a matching port.

This is required to allow the 'chromium-' prefix to be removed from port names
independently of renaming the fallback directories.

BUG=231188
R=ojan

Review URL: https://chromiumcodereview.appspot.com/16888010

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@152596 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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