webpages_playback.py: Support storing to local directories

webpages_playback.py: Support storing to local directories

Make webpages_playback support storing to local directories as an
alternative to Google Storage.

Rename argument --dest_gsbase to --data_store=<location>. Make this parameter
accept either gs:// url or directory for local file system storage.

Rename argument --upload_to_gs to --upload.

Implement file existence check, download and upload for the file
operation.
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/737123005