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This CL updates all deprecated methods of base::Value and
base::JSONReader in OAuth2MintTokenFlow to use recommended APIs.

Bug: 1026237
Change-Id: I4e15970bc6215b74c9d4cea2d96f0923e1c7b878
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1940177Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719910}