Reland "[Context Aware Signals] Fix invalid encoding key"

This is a reland of ce4a4d29552e1c2d38f17cf5dcc90adf3e8c69c6.

Original change's description:
> Public key for Context Aware Signals for Browser had wrong
> exportation method (ExportRawPublicKey). The format should be DER.
>
> This change make use of the method ECPrivateKey::ExportPublicKey
> instead of ECPrivateKey::ExportRawPublicKey. Also all the code
> related to the PEM format is removed.
>
> Additional refactor is done like:
> - Adding a method `LoadKeyPair`
> - Improving testings
>
> Bug: 1212786,1176175 , b/190055390 , b/190053957
> Change-Id: I5ec94ab5cb4f122c623a3963b8c03291a47768c1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2940169
> Commit-Queue: Martin Rodriguez <rodmartin@google.com>
> Reviewed-by: David Benjamin <davidben@chromium.org>
> Reviewed-by: Roger Tawa <rogerta@chromium.org>
> Reviewed-by: Alice Gong <alicego@google.com>
> Cr-Commit-Position: refs/heads/master@{#893873}

Bug: 1212786,1176175 , b/190055390 , b/190053957
Change-Id: Ifcc97d1466e7117cd071f9c8ea702ce16c850e80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2976402
Reviewed-by: Alice Gong <alicego@google.com>
Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Commit-Queue: Martin Rodriguez <rodmartin@google.com>
Cr-Commit-Position: refs/heads/master@{#894698}