WebView Log Verbosifier is an empty dummy app (in fact, it cannot be launched). However, if this app is installed, WebView will log the active field trials and CommandLine flags, for debugging/QA purposes. A dummy app is used because it can be installed on any device (including user builds, where field trials are still relevant).
autoninja -C out/Default webview_log_verbosifier_apk out/Default/bin/webview_log_verbosifier_apk install
Alternatively, if you do not have a fully set up Chromium build environment, you can build with gradle:
ANDROID_SDK_ROOT=/path/to/sdk ./gradlew build
grep the logcat like so:
adb logcat | grep -i 'Active field trial' # Field trials, one per line adb logcat | grep -i 'WebViewCommandLine' # CommandLine switches, one per line adb logcat | grep -iE 'Active field trial|WebViewCommandLine' # Both
Then just start up any WebView app.
When you're done investigating flags/field trials, you can disable the logging by uninstalling the app: