Add dry-run mode, activated by env var POLLER_DRY_RUN.
Add dry-run mode, activated by env var POLLER_DRY_RUN.

R=nodir@chromium.org,kmg@chromium.org,iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/424543007

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@287815 0039d316-1c4b-4281-b951-d872f2087c98

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