SendTabToSelf: Encrypt page_context in the proto

This change introduces an `encrypted_page_context` field in
`SendTabToSelfSpecifics`.

The encryption and decryption is implemented similarly to how it's done
for Passwords and OutgoingPasswordSharingInvitations:

The `DataTypeWorker` is handles the encryption and decryption using the
Nigori keys. Before committing, the `page_context` is encrypted and
stored in `encrypted_page_context`, and the plaintext field is cleared.
Upon receiving updates, the `DataTypeWorker` decrypts
`encrypted_page_context` and populates the `page_context` field before
passing it to the bridge.

However, an important difference is that for Passwords and
OutgoingPasswordSharingInvitations, the custom encryption *replaces* the
usual full-specifics encryption. For SendTabToSelf, on the other hand,
the custom field-level encryption is only applied when full-specifics
encryption is *not* enabled.

Bug: 493304635
Change-Id: I73e399034c275acffa051efe54e16fb0ce38976b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7722724
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1618262}