Add Result FrameDecryptorInterface::Decrypt

This change adds FrameDecryptorInterface::Result to the FrameDecryptorInterface
API. Result contains a Status and bytes_written. This removes requiring out
parameters from the API and provides a simpler status return code for the
function. This is in response to comments suggested here:
https://webrtc-review.googlesource.com/c/src/+/131358

int FrameDecryptorInterface::Decrypt() will be removed in a follow up CL.

Bug: webrtc:10512
Change-Id: I47f19f154d1d8430acd6e4a6f433ab24c455fd51
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/131362
Commit-Queue: Benjamin Wright <benwright@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27470}