AW: add metric to track baseUrl scheme

This adds an UMA metric to track loadDataWithBaseURL's baseUrl scheme.
This logs if the URL is empty, if the scheme is one of several
well-known values, or if the scheme is some unknown value (possibly a
custom URL scheme).

This also adds integration tests to verify we record metrics.

R=isherman@chromium.org, torne@chromium.org

Bug: 901581
Test: run_webview_instrumentation_test_apk \
Test: --gtest_filter=LoadDataWithBaseUrlTest#testBaseUrlMetrics_*
Change-Id: Iaedf6836daeb185da7b1b26145d30fb46a363d21
Reviewed-on: https://chromium-review.googlesource.com/c/1321868
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607281}
4 files changed