* fix: use render client id to track deleted render process hosts
Instead of relying on OS process id, which may not be unique
when a process is reused, we rely on the renderer client id
passed by the content layer when starting the renderer process
which is guaranteed to be unique for the lifetime of the app.
* fix: store context id as int64_t
Ensuring that it doesn't wrap easily with a large number
of context creation on some malformed web pages.
RenderProcessHost switch can happen between ipc calls when
speculative process are invvolved, which will lead to deletion
of entries on current context. Use OS process handles to
uniquely associate a destruction handler for a render process.
