I'm not sure if that's quite right? I think the fallback should be set on cross-process navigation? Anytime the FrameSinkId changes, basically, right? That's a different IPC perhaps.

We still don't need an IPC. Whenever the FrameSinkId of the primary surface changes we can just update the fallback to be the first primary surface with the old FrameSinkId. ChildFrameCompositingHelper can automatically do this.

I like it!

I think it's a good idea to work on this task after https://chromium-review.googlesource.com/c/chromium/src/+/1157404 is merged.

Won't the SurfaceRange grow unbounded if you never update the start SurfaceId?

kylechar: That's not really an issue anymore. We only need to update the start of the range to limit the earliest SurfaceId that can be embedded.

Also, we will not end up with a lot of surfaces even if the range is huge, because garbage collection will keep on removing old surfaces so the number of active surfaces per framesinkid is expected to be small.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/383a1a1f7367e5ffdc717b304ab1f6f8c4cc2336

commit 383a1a1f7367e5ffdc717b304ab1f6f8c4cc2336
Author: akaba <akaba@google.com>
Date: Wed Aug 22 18:45:43 2018

Don't send OnFirstSurfaceActivation to renderer processes when surface sync is active.

This CL prevents the following messages from being sent to renderers when surface
sync is enabled:

FrameMsg_FirstSurfaceActivation
BrowserPluginMsg_FirstSurfaceActivation

This is because the extra IPC for updating is not needed anymore after fallback surfaces
are fully optional.

Bug:  870452 
Change-Id: I4a96fc759e230031d82ed4feb3c7c5697ca3c3c8
TBR: tsepez@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1183588
Commit-Queue: Andre Kaba <akaba@google.com>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585186}
[modify] https://crrev.com/383a1a1f7367e5ffdc717b304ab1f6f8c4cc2336/content/browser/browser_plugin/browser_plugin_guest.cc
[modify] https://crrev.com/383a1a1f7367e5ffdc717b304ab1f6f8c4cc2336/content/browser/frame_host/cross_process_frame_connector.cc