services/tracing: Make stopping a failed tracing session a no-op

If a tracing session failed to start, stopping it shouldn't do anything
either.

Bug: 1139845
Change-Id: Ib68f787ff009d2c81fbd928a4cf625f290f65b2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487101
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Auto-Submit: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818874}

services/tracing/public/cpp/perfetto/perfetto_tracing_backend.cc

@@ -389,7 +389,8 @@ class ConsumerEndpoint : public perfetto::ConsumerEndpoint,
 
   void DisableTracing() override {
     DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
-    tracing_session_host_->DisableTracing();
+    if (tracing_session_host_)
+      tracing_session_host_->DisableTracing();
   }
 
   void Flush(uint32_t timeout_ms, FlushCallback callback) override {