And in L we don't call WebViewChromiumFactoryProvider, we call its ctor, so the class that will have to be a singleton is not the WebViewChromiumFactoryProvider itself, but something it owns (like WebViewChromiumAwInit added in
https://chromium-review.googlesource.com/c/chromium/src/+/895942

I'll have to scope out exactly what the support library glue needs access to before deciding which class to turn into a singleton (if we want access to existing glue-layer adapters we might want another class to hold those rather than WebViewChromiumAwInit..).

> And in L we don't call WebViewChromiumFactoryProvider.create, we call its ctor

In practice, there's still only one instance of WebViewChromiumFactoryProvider even though you can create more. So you could say have the constructor set a global variable, and have a getInstance method that returns that.

although that only works if support lib can force android to create WebViewChromiumFactoryProvider first before trying to use it

Android will always create WebViewChromiumFactoryProvider first - the way the support library hooks into the webview apk classloader will trigger the code in the framework (WebViewFactory) that reflects into WebViewChromiumFactoryProvider.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3854eccb09e28bb9316aa1305ad96aa6f8c8b199

commit 3854eccb09e28bb9316aa1305ad96aa6f8c8b199
Author: Gustav Sennton <gsennton@google.com>
Date: Tue Feb 06 02:13:59 2018

Add a singleton from where to fetch the WebViewChromiumFactoryProvider.

The WebView support library glue needs a way to fetch the
WebViewChromiumFactoryProvider, this CL adds a mechanism for keeping
track of our only instance of WebViewChromiumFactoryProvider.

Bug:  808417 

Change-Id: I37c76d82b01004753d962ade6874184b28eaa54b
Reviewed-on: https://chromium-review.googlesource.com/901624
Commit-Queue: Gustav Sennton <gsennton@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534588}
[modify] https://crrev.com/3854eccb09e28bb9316aa1305ad96aa6f8c8b199/android_webview/glue/java/src/com/android/webview/chromium/WebViewChromiumFactoryProvider.java

oh one more note for support lib, you probably want @UsedByReflection on things that will be used by reflection, so proguard doesn't obfuscate or optimize away the things you need

Ah, thanks Bo! Filed  crbug.com/809471