AW: update net docs for WebView's feature support

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}

net/docs/supported-projects.md

@@ -73,10 +73,12 @@ For official Android devices, WebView has the following capabilities.
   * **Automatic Updates**: Varies. Updates are made available on the Android
     App Store, but users must explicitly choose to update. As such, the
     rate of update varies much more than for the Chromium browser.
-  * **Command-line Flags**: No
-  * **Field Trials (Finch)**: No
-  * **Enterprise Policy**: No
-  * **User Metrics (UMA)**: No
+  * **Command-line Flags**: No for production devices, [yes for userdebug
+    devices](https://chromium.googlesource.com/chromium/src/+/HEAD/android_webview/docs/commandline-flags.md)
+  * **Field Trials (Finch)**: Yes, [with
+    caveats](https://g3doc.corp.google.com/analysis/uma/g3doc/finch/platforms.md?cl=head)
+  * **Enterprise Policy**: Yes, with caveats (TODO(rsleevi): document caveats)
+  * **User Metrics (UMA)**: Yes, [with caveats](http://go/clank-webview/uma)
   * **Component Updater**: No
 
 ## `//content` Embedders