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This tidies the code up a bit, and I use these interfaces in a subsequent changelist (for EC keys).

The vision is that jwk.h/jwk.cc should just define these two helpers, and all the other algorithm-specific logic (EC keys, RSA keys, AES keys, HMAC keys etc..) is built using JwkReader/JwkWriter, and lives in the algorithm-specific files.

BUG=399094

Review URL: https://codereview.chromium.org/687063002

Cr-Commit-Position: refs/heads/master@{#302697}