The RunIfWaitingForDebugger command was made stable, breaking this test
which is specifically trying to check the behavior of an experimental
command. Switch to another experimental command.

(The API should have remained compatible even if the command was made
stable. This bug will be fixed separately.)

BUG=640919
NOTRY=true

Review-Url: https://codereview.chromium.org/2277193002
Cr-Commit-Position: refs/heads/master@{#414385}