TL;DR:

RenderWidgetHostImpl::GetSnapshotFromBrowser is used exclusively by dev tools' PageHandler::CaptureScreenshot(), which is used by telemetry.

The implementation ab(uses) LatencyInfo and it falls apart in Mus. Basically GetSnapshotFromBrowser() initiates a redraw request, and then uses LatencyInfo to have RenderWidgetHost notified when a GPU Swap occurs for that redraw request. The ideal way of fixing this would be to find another way for RenderWidgetHost to be notified when a GPU Swap happens, and then just to get rid of BROWSER_SNAPSHOT_FRAME_NUMBER_COMPONENT altogether.

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More info:

There's some awkward plumbing where latency compontent id is set to be (process_id << 32 | routing_id), and then GPU Swap notifications is routed based on this (see RenderWidgetHostImpl::OnGpuSwapBuffersCompleted). There's some discussion regarding this being hacky in the original CL: https://codereview.chromium.org/23694031 (see piman's and jamesr's comments). 

Reasons this falls apart in Mus:
1. The actual snapshot is taken in RenderWidgetHostImpl::OnGpuSwapBuffersCompletedInternal, which won't be called in Mus.
2. Deducing which RenderWidgetHost requested the screenshot is done based on latency component id (see RenderWidgetHostImpl::OnGpuSwapBuffersCompleted), which is set in RenderWidgetHostLatencyTracker::AddLatencyInfoComponentIds, called from RenderWidgetHostLatencyTracker::OnSwapCompositorFrame, which also won't be called in Mus.