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.
After after using `processId-contextCounter` as contextId, it may happen
that contexts in different WebContents sharing the same renderer process
get the same contextId. Using webContentsId as part of key in
ObjectsRegistry can fix this.
After the page does navigations, garbage collection can still happen in
the old context. This commit changes to store references to remote objects
by _pages_, instead of by _WebContents_.
