runtest.py: Unify --pass-build-dir handling, always call ConvertBuildDirToLegacy
runtest.py: Unify --pass-build-dir handling, always call ConvertBuildDirToLegacy

No intended behavior change.

BUG=294387

Review URL: https://codereview.chromium.org/31243003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@229718 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build