thermald: remove base::JSONReader::.*Deprecated methods

This is a libchrome post r680000 uprev cleanup.
This also adds TODO for planned base::Value cleanup (crbug.com/1099111).

BUG=chromium:1054279
TEST=cros_run_unit_tests --board=mistral --packages thermald

Change-Id: I1a03b9bb409902b5e3682058248fdc5086a4a2c9
Reviewed-on: https://chrome-internal-review.googlesource.com/c/chromeos/thermald/+/3119793
Tested-by: Grace Cham <[email protected]>
Commit-Queue: Grace Cham <[email protected]>
Reviewed-by: Hidehiko Abe <[email protected]>
2 files changed