Simple Cache: Make stream 2 optional

Make the third stream optional, since it's usually empty.

BUG=280592

Review URL: https://chromiumcodereview.appspot.com/22511002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224674 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224674