Rename BrowserPluginGuest::GetGuestProxyRoutingID()

This function returns the RenderView routing_id, not a RenderFrameProxy
routing id. Rename it to be less confusing.

1. s/GetGuestProxyRoutingID/GetGuestRenderViewRoutingID/g
2. s/guest_proxy_routing_id_/guest_render_view_routing_id_/g
3. Make GetGuestProxyRoutingID() to be private.

Bug: None.
Change-Id: If98da438ae89cd8d702832b05d142d2c5fed7794
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617831
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661654}
2 files changed