This *almost* works. After the code change (demo CL: http://crrev/c/1056142), I get one compile error:

../../android_webview/support_library/callback/java/src/org/chromium/support_lib_callback_glue/SupportLibWebViewContentsClientAdapter.java:58: error: incompatible types: WebViewClient cannot be converted to BoundaryInterface
                BoundaryInterfaceReflectionUtil.createInvocationHandlerFor(possiblyCompatClient);

This makes sense: possiblyCompatClient is a WebViewClient. At this point in the code, we know it's a WebViewClientCompat (and therefore inherits the corresponding BoundaryInterface). However, it's a WebViewClientCompat defined in the *other* ClassLoader, and therefore is not considered to be a BoundaryInterface as defined in *this* ClassLoader.

I don't know if there's a way around this--even if we do trickery to cast this to a BoundaryInterface[app ClassLoader], it still won't be considered a BoundaryInterface[chromium ClassLoader].

I have not yet attempted cherry picking this change to the support library side, so that may also pose issues.

I'm going to close this for now, due to the compile error mentioned in comment 1. I don't think this error is unsolvable, but I think any solution would overly complicating the code, outweighing potential benefit.