In my_activity.py, multiple --last_week for more weeks
In my_activity.py, multiple --last_week for more weeks

Also, if one --last_week option is given, the date range should be
strictly one week before when without the option.

BUG=

Review URL: https://codereview.chromium.org/1086083007

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@295052 0039d316-1c4b-4281-b951-d872f2087c98

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