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It used to return the routing_id of the RenderFrameProxyHost for which a
RenderFrameProxy was initialized. This is a bit arbitrary and as a
result, among 6 callers, nobody was using it, except BrowserPluginHost.
It is probably better to return nothing instead.

Bug: None.
Change-Id: Idcc46d469f3eae53cabd5d09fa29636165804b69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614177
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661320}