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/1161518Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#54903}