AW: document flag UI and update OWNERS

No change to logic. This documents WebView's flag UI (including benefits
of exposing flags through the UI) and adds myself as an OWNER for
ProductionSupportedFlagList.

Bug: 1058505
Test: tools/md_browser/md_browser.py android_webview/docs/flag-ui.md
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Ie8726a19ca04c1ee3b4380508c05c3fd47b492f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088399Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746970}

android_webview/docs/commandline-flags.md

@@ -3,9 +3,11 @@
 ## Can I apply commandline flags on my device?
 
 *** note
-**Note:** WebView only permits toggling commandline flags on devices/emulators
-running a debuggable Android OS image. **Most users cannot apply commandline
-flags, because they're using devices with production Android images.**
+**Note:** WebView only permits toggling arbitrary commandline flags on devices/emulators
+running a debuggable Android OS image. **Most users cannot follow these
+instructions**, because they're using devices with production Android images.
+
+If you need to toggle flags on **production devices**, see [this guide](flag-ui.md).
 ***
 
 You can check which Android image you have on your device with the following:

android_webview/docs/flag-ui.md

+# Flag UI
+
+While WebView supports [toggling arbitrary flags](commandline-flags.md) on
+debuggable devices, we also support toggling a curated set of experimental
+flags/features on production Android devices. We expose these features as part
+of WebView's on-device DevTools. This is similar to Chrome's `chrome://flags`
+tool.
+
+![WebView flag UI](images/webview_flag_ui.png)
+
+## Using the flag UI
+
+This feature is launching soon! Until then, Googlers can follow instructions in
+[this email thread](http://shortn/_Whbt0gWDS9).
+
+<!-- TODO(ntfschr): fill this out when Dev UI launches (https://crbug.com/1002589)-->
+
+## Adding your flags and features to the UI
+
+If you're intending to launch a feature in WebView or start a field trial (AKA
+Finch experiment), we **highly encourage** you to [add to the
+list](/android_webview/java/src/org/chromium/android_webview/common/ProductionSupportedFlagList.java)
+(ex. [CL](https://crrev.com/c/2008007), [CL](https://crrev.com/c/2066144)).
+
+Exposing your feature this way has several benefits:
+
+- This improves the manual test process. Testers can enable your feature with a
+  button click instead of an adb command.
+- Typo-free: someone could mistype a flag or feature name in the commandline,
+  but this UI ensures flag names are always spelled correctly.
+- Because this works on production Android devices, test team can validate your
+  feature on devices from other OEMs.
+- You (and teammates) can dogfood your feature.
+- If users or third-party app developers report bugs, this UI is the only way
+  they can toggle your feature to help root-cause the regression.
+
+## See also
+
+- [Design doc](http://go/webview-dev-ui-flags-design) (Google-only)

android_webview/java/src/org/chromium/android_webview/common/OWNERS

+per-file ProductionSupportedFlagList.java=ntfschr@chromium.org