This is a reland of bf5ea813

Original change's description:
> [tracing] allow dynamic control of tracing
> 
> If the trace_buffer_ was null, we were returning a pointer to a static
> flag back that permanently disabled that particular trace point.
> 
> This implied an assumption that tracing will be statically enabled at
> process startup, and once it is disabled, it will never be enabled
> again. On Node.js side we want to dynamically enable/disable tracing as per
> programmer intent.
> 
> Change-Id: Ic7a7839b8450ab5c356d85e8e0826f42824907f4
> Reviewed-on: https://chromium-review.googlesource.com/1161518
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
> Cr-Commit-Position: refs/heads/master@{#54903}

Change-Id: Ie2a9ebbaf59d3be1918864cc3c192671fb5550c3
Reviewed-on: https://chromium-review.googlesource.com/1188529
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56007}