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This patch reverts the previous revert from Jim and also add a
variable user_priv in the FrameWorker to save the user_priv
passed from the application. In the decoder_get_frame function,
the user_priv will be binded with the img. This change is needed
or it will fail the unit test added here:
https://gerrit.chromium.org/gerrit/#/c/70610/

This reverts commit 9be46e45.

Change-Id: I376d9a12ee196faffdf3c792b59e6137c56132c1