Can we use proguard to remove the unnecessary onResourceLoaded()? 

Proguard has no concept of webview-reachable vs chrome-reachable, so I don't think we could.

One thing I can't remember is that if Android platform requires or assumes all resources's package ID be changed, if so we have to change the package id for all resources of Monochrome even they are not actually used, otherwise it might conflicts with application's resources.

re #3 - this doesn't apply to the ids as they exist within resources.arsc. This applies only to the generated .java code. So, I don't see how this could affect anything external to our apk.

OK, I may not understand how the new way works

Here is what webview's ./gen/android_webview/glue/glue_resource_rewriter.srcjar looks like:

    public static void rewriteRValues(final int packageId) {
        org.chromium.android_webview.R.onResourcesLoaded(packageId);
        org.chromium.components.web_contents_delegate_android.R.onResourcesLoaded(packageId);
        org.chromium.content.R.onResourcesLoaded(packageId);
        android.support.v7.mediarouter.R.onResourcesLoaded(packageId);
        org.chromium.ui.R.onResourcesLoaded(packageId);
        org.chromium.media.R.onResourcesLoaded(packageId);
        org.chromium.components.autofill.R.onResourcesLoaded(packageId);
        com.google.android.gms.R.onResourcesLoaded(packageId);
        android.support.v7.appcompat.R.onResourcesLoaded(packageId);
    }


One notable here is the inclusion of appcompat, which I wouldn't have expected. Looks like it's due to a single call to AppCompatResources.getDrawable() in ui_java:
https://cs.chromium.org/chromium/src/ui/android/java/src/org/chromium/ui/DropdownAdapter.java?type=cs&q=android.support+file:%5Esrc/ui/android/java/+package:%5Echromium$&l=10

The addition of play services for webview also adds a lot of resources (and thus IDs that are re-written, 99% of which are unused).

Looking at .proguard.jar.usage files, it does appear that proguard removes the unused onResourcesLoadeded() calls and inlines IDs for the R classes not listed in rewriteRValues. So... I don't think there's anything that can be done here in terms of whitelisting until we figure out proper unused resource removal (bug 636448). If we had that, then we would know which support resources are used only by webview and not chrome, and use non-final IDs only for these.

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This got implemented.