See  crbug.com/788177  for context on boundary interfaces.

This bug is about generating code for each support library class to implement those interfaces, or call into them, depending on whether instances of the class are created in the support library / app, or in the webview apk.

As an example, see
https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/support/+/3217743/3

where WebSettingsImpl:
https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/support/+/3217743/3/webkit/src/main/java/androidx/webkit/WebSettingsImpl.java
implements WebSettingsCompat (app-facing) by calling into the boundary interface WebSettingsInterface.

vs. VisualStateCallbackImpl:
https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/support/+/3217743/3/webkit/src/main/java/androidx/webkit/VisualStateCallbackImpl.java
which implements VisualStateCallbackInterface to call into a VisualStateCallback object created by the app.

How to determine which class should implement the app-facing interface vs. the boundary interface is described here:  crbug.com/788174