The sandbox option allows multiple webContents in one renderer process, so using
the only the renderer id to identify WebContents instances is no longer an
option.
WebContents::GetID now returns a 64-bit integer, which is composed of both the
process id(high 32), and the RenderViewHost routing id(low 32). Also add a
`GetProcessID` that retrieves the renderer process id, a requirement in some of
our javascript code.
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