--profile should imply --test-runner-count=1 in run-perf-tests

--profile should imply --test-runner-count=1 in run-perf-tests
https://bugs.webkit.org/show_bug.cgi?id=130375

Reviewed by Benjamin Poulain.

Use the test runner count of 1 when --profile is specified but not --test-runner-count.

* Scripts/webkitpy/performance_tests/perftestsrunner.py:
(PerfTestsRunner._parse_args): Use -1 as the default value.
(PerfTestsRunner._collect_tests): If --test-runner-count is set (i.e. it's a positive value),
then use that value. Otherwise, if it's never set and --profile is present, test-runner-count
should be 1.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165782 268f45cc-cd09-0410-ab3c-d52691b4dbfc