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No change to logic, only a docs update.

This updates supported-projects.md to reflect WebView's most recent
feature support, and points out where WebView differs from Chrome.

 * We've supported CLI flags ~forever. These are useful for local
   debugging, even if they can't be used on production devices. This
   links to upstream docs.
 * Finch support launched this year. It still has some caveats, which
   are described by the linked internal documentation.
 * Per an offline discussion with rsleevi@, WebView has enterprise
   policy support, although this is poorly documented.
 * We've supported UMA for over a year. This links to internal
   documentation describing caveats.

R=rsleevi@chromium.org

Test: None
Change-Id: I088880eb9c5bf78e8c8f9574f62b8acac94073bf
Reviewed-on: https://chromium-review.googlesource.com/c/1392255Reviewed-by: Paul Miller <paulmiller@chromium.org>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619828}