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This change does a couple of things to ensure we don't track any
unnecessary data for checker-imaging:

1) During discardable image metadata generation, the decoding mode only
   needs to be tracked if the image is discardable.
2) We don't need to store any decoding mode metadata if checker-imaging
   is disabled.

R=ericrk@chromium.org

Bug: 1003886
Change-Id: I2802af3bcf69beb970b98a6c179032a72b588f2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1973152
Commit-Queue: Khushal <khushalsagar@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726685}