Webapk install function that will be used in chrome service for weblayer

The Chrome service will receive the serialized webapk-proto from the
weblayer client, so a function in webapk_installer was added that skips
the step of building that proto and installs the webapk directly.

Tests: ninja -C out/Default unit_tests && \
          ./out/Default/bin/run_unit_tests \
          --gtest-filter "*WebApkInstallerTest*"

Bug: 1101999
Change-Id: I3a0bafea8e07dd3135c01e1384d8108c8b9a47a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3501890


Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Glenn Hartmann <hartmanng@chromium.org>
Reviewed-by: Rohit Agarwal <roagarwal@chromium.org>
Commit-Queue: Susanne Westphal <swestphal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#979732}