Block LayoutRPC until the first reflow.

Fixes #3232.

Previously, LayoutRPC calls would be allowed before Layout has had a chance to
even run. This would mean there'd be a small chance of getClientBoundingRect
calls failing the script task.

This patch takes the RPC lock until reflow has happened at least once,
preventing such races. This behavior was reported as issue #3232. @glennw has
confirmed that this fixed the failure that he saw.