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Now all the thing that the class does is to delete request objects.
The lifetime management duty is moved to RequestSender.
* NotifyStart => no op.
* NotifyFinish => sender_->RequestFinished.

BUG=164098

Review URL: https://chromiumcodereview.appspot.com/17175017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@208011 0039d316-1c4b-4281-b951-d872f2087c98