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Currently it just checks the filename duplication and adding numbers (1),(2),... in
case of duplication.  But this doesn't work well if the file will be stored into GoogleDrive,
since file operations are not atomic there.

This CL does the followings:
- the screenshot taker accepts "taking a screenshot for each root window" requests
  rather than taking a screenshot for the specified window
- adding the number in case of multiple root-windows, in screenshot taker side
- change the screenshot interval from 500msec -> 1sec, to avoid the duplication
- remove the existing filename duplication checking code, which is no longer necessary

BUG=140749

Review URL: https://chromiumcodereview.appspot.com/10827193

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150758 0039d316-1c4b-4281-b951-d872f2087c98