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Input predictor's GeneratePrediction used to return a boolean for
whether the prediction exist and also a InputData class contains the
prediction results. This is unnecessary.
This CL makes:
1. GeneratePrediction returns a unique_ptr of InputData class. If
prediction exist, it's the predict result; otherwise, it's nullptr.
2. No need to check HasPrediction before GeneratePrediction.
It should be decided by predictor itself and returns result or nullptr
accordingly.

Change-Id: Id5e092dc98f4c0fa4e66ac709e05fbf7cae8c734
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849173
Commit-Queue: Ella Ge <eirage@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708030}