Escape commas in Chrome startup arguments for telemetry tests on CrOS.

BUG=239161
TEST=run_tests pass for tests with commas in startup arguments
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/15313004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@200921 0039d316-1c4b-4281-b951-d872f2087c98