AW docs: document Dev UI

No change to logic, only docs.

This adds general documentation for how to launch Dev UI, and provides
information for how Dev UI interacts with Finch.

Fixed: 1058571
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: Icc7234807935d764886a66305ff8cd5653701f5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2123334Reviewed-by: Hazem Ashmawy <hazems@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755053}

android_webview/docs/flag-ui.md

 # Flag UI
 
+[TOC]
+
 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
@@ -8,12 +10,89 @@ tool.
 
 ![WebView flag UI](images/webview_flag_ui.png)
 
+## Launching WebView DevTools
+
+The flag UI is part of "WebView DevTools," an on-device tool that ships with
+device's WebView implementation. You can launch WebView DevTools by any of the
+following:
+
+### Launcher icon on pre-stable channels (preferred)
+
+The best way to launch WebView DevTools is to [download WebView Dev or WebView
+Canary](prerelease.md). These channels will have a launcher icon which will
+launch WebView DevTools.
+
+*** note
+**Note:** the WebView DevTools icon does not appear by default on Android 7
+through 9 (Nougat/Oreo/Pie). To enable the launcher icon, first [change your
+WebView provider](prerelease.md#Android-7-through-9-Nougat_Oreo_Pie) and then
+launch the same Chrome channel or any WebView app (ex. [WebView shell
+browser](webview-shell.md), or open an email in Gmail).
+***
+
+### Launch via adb
+
+If you have adb installed, you can connect your Android device to launch
+DevTools:
+
+```sh
+adb shell am start -a "com.android.webview.SHOW_DEV_UI"
+```
+
+### Launch via WebView Shell
+
+Newer versions of [WebView shell](webview-shell.md) have a menu option to launch
+WebView DevTools. If your copy of WebView shell doesn't have this option, you
+may need to rebuild it yourself.
+
 ## Using the flag UI
 
-This feature is launching soon! Until then, Googlers can follow instructions in
-[this email thread](http://shortn/_Whbt0gWDS9).
+Once you've launched WebView DevTools, tap the "Flags" option in the bottom
+navgation bar. You can scroll through the list to find your desired feature/flag
+(ex. "highlight-all-webviews"), tap the dropdown (look for "Default"), and tap
+"Enabled" in the dialog popup. You can enable (or disable) as many flags as you
+need.
+
+*** promo
+**Tip:** enabling "highlight-all-webviews" (which tints all WebView objects
+yellow) in addition to your desired flag is a great way to verify apps have
+picked up WebView flags.
+***
+
+Kill and restart WebView apps so they pick up the new flags.
+
+When you're done, open the notification tray and tap the WebView DevTools
+notification to go back to the flag UI. Tap "Reset all to default" and kill and
+restart WebView apps to go back to the default behavior.
+
+### Overriding variations/Field Trials
+
+Like Chrome, WebView supports A/B experiments and feature rollouts through
+variations (AKA "field trials" or "Finch"). The flag UI can override the field
+trial config, either to **enable** an experimental feature to ensure your app
+works correctly, or to **disable** an experiment to determine if this is the
+root cause for a WebView behavior change breaking your app. Simply tap "Enabled"
+or "Disabled" in the UI; "Default" means WebView will pick up the random field
+trial experiment.
+
+If you find an experiment is the root cause for app breakage, please [file a
+bug](https://bugs.chromium.org/p/chromium/issues/entry?template=Webview+Bugs),
+mention which experiment, and link to your app's Play Store page for our team to
+investigate.
+
+### Accelerating field trial config download
+
+You can also use the flag UI to download new field trial configs ("seeds") more
+quickly, to verify the next seed will fix app breakage. Enable all of the
+following:
+
+* `finch-seed-expiration-age=0`
+* `finch-seed-min-update-period=0`
+* `finch-seed-min-download-period=0`
+* `finch-seed-ignore-pending-download`
 
-<!-- TODO(ntfschr): fill this out when Dev UI launches (https://crbug.com/1002589)-->
+Restart your app, kill it, and restart it a second time. Your app should be
+running with the latest WebView variations seed.
 
 ## Adding your flags and features to the UI