AW docs: link to public flash tool

Android recently launched a public version of Android Flash Tool. This
updates our device setup instructions to refer external contributors to
use the public tool.

Bug: 918221
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I4956ee9633242e6c727f9dfd7ccf378a2c44fb11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2147691
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758656}

android_webview/docs/device-setup.md

@@ -42,13 +42,16 @@ developer hurdles.
 
 ## Physical device
 
-### Flash a prebuilt image (Googler only)
+### Flash a prebuilt image
 
 Googlers can consult internal instructions
 [here](http://go/clank-webview/building-webview/device-setup).
 
-Unfortunately, Google does not offer prebuilt `userdebug`/`eng` images for
-external contributors.
+External contributors can flash a prebuilt userdebug image (based off
+aosp-master) onto a Pixel device with [Android Flash
+Tool](https://flash.android.com/welcome?continue=%2Fcustom). This requires a
+browser capable of WebUSB (we recommend the latest Google Chrome stable
+release).
 
 ### Building AOSP yourself (hard way)