runtest.py: Add --perf-config flag.
runtest.py: Add --perf-config flag.

This flag makes it possible to pass a --perf-config
flag to runtest.py instead of relying on the "perf_config"
factory property.
This will make it possible to use this properly with recipes.

BUG=None
TEST=Tested manually using:
Error cases:
scripts/tools/runit.py scripts/slave/runtest.py --perf-config=foo
scripts/tools/runit.py scripts/slave/runtest.py --perf-config={'foo':}
scripts/tools/runit.py scripts/slave/runtest.py --perf-config={'foo':'bar'}
Working cases:
scripts/tools/runit.py scripts/slave/runtest.py --perf-config={}
scripts/tools/runit.py scripts/slave/runtest.py --perf-config="{'foo':'bar'}"

Review URL: https://codereview.chromium.org/271703013

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@270393 0039d316-1c4b-4281-b951-d872f2087c98

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