Talked with Ali and a theory is that windows delivers scrollwheel events to the compositor thread (where we try to impl-scroll but fail since it's broken), and there's nothing that forwards these events on to the main thread. Scrolling with the drag handles does work on windows.

This ended up being related to layer id lookups being wrong[1] which wkorman noted is wrong [2]. I'm going to fix this as a followup to [3].

[1] https://cs.chromium.org/chromium/src/cc/trees/layer_tree_host_impl.cc?q=tryscroll&sq=package:chromium&l=2561
[2] https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp?q=propertytreemana&sq=package:chromium&l=269
[3] https://codereview.chromium.org/2657863004

Fixed