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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}