Signin: Clean up GN structure of MutablePO2TS and webdata

In preparation for the code restructuring of //components/signin, this
CL moves mutable_profile_oauth2_token_service_delegate* from the
//components/signin/core/browser target to the
//components/signin/core/browser:internals target where it belongs. This
change has the ripple effect of requiring
//components/signin/core/browser/webdata to be separated out of
//components/signin/core/browser in GN, as MutablePO2TS depends on the
webdata. That change is also conceptually correct as webdata is
conceptually layered below all of the rest of //components/signin.

Bug: 952788
Change-Id: I309512f4904157b1881ec9860bdec2c17f371245
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1681701
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673306}
9 files changed