Isn't fallback being set to this SurfaceId via:

client_->DelegatedFrameHostGetLayer()->SetFallbackSurfaceId(
      surface_info.id());

?

Can you link code search? 

Yeah, https://cs.chromium.org/chromium/src/content/browser/renderer_host/delegated_frame_host.cc?sq=package:chromium&g=0&l=369

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/241d50b9c1c7b7a625eba75314afde3ee42200bb

commit 241d50b9c1c7b7a625eba75314afde3ee42200bb
Author: Saman Sami <samans@chromium.org>
Date: Tue Sep 18 00:25:28 2018

Evict the last activated surface instead of fallback

Last activated surface and fallback are no longer the same thing.
Taking the SurfaceId from the SurfaceLayer's fallback no longer works
properly.

Bug:  874983 
Change-Id: I772e69f7e009935086e090c859c1300caec6283e
Reviewed-on: https://chromium-review.googlesource.com/1225990
Commit-Queue: Saman Sami <samans@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591894}
[modify] https://crrev.com/241d50b9c1c7b7a625eba75314afde3ee42200bb/ui/android/delegated_frame_host_android.cc