Register tracing data sources early if SystemTracing is enabled.

This ensures that all processes are available for tracing since the data
sources will be registered. Before only the browser process registered
data sources, and thus you had to run a local trace before a system
trace to get data from the renderer. This fixes that so all processes
will register the TraceEvent and TraceSampler data sources
(TraceMetadata remains browser only).

Since the trace processor doesn't support proto traces yet I can't write an
integration tests for this behaviour change.

Bug: 979576,1007275
Change-Id: Ibb5aed2725ed2eae3672a3fddd5a31db7804f247
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1821602Reviewed-by: Stephen Nusko <nuskos@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Stephen Nusko <nuskos@chromium.org>
Auto-Submit: Stephen Nusko <nuskos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699823}

services/tracing/public/cpp/trace_startup.cc

@@ -13,6 +13,7 @@
 #include "components/tracing/common/tracing_switches.h"
 #include "services/tracing/public/cpp/perfetto/trace_event_data_source.h"
 #include "services/tracing/public/cpp/stack_sampling/tracing_sampler_profiler.h"
+#include "services/tracing/public/cpp/trace_event_agent.h"
 #include "services/tracing/public/cpp/tracing_features.h"
 
 namespace tracing {
@@ -85,6 +86,8 @@ void InitTracingPostThreadPoolStartAndFeatureList() {
   // Below are the things tracing must do once per process.
   TraceEventDataSource::GetInstance()->OnTaskSchedulerAvailable();
   if (base::FeatureList::IsEnabled(features::kEnablePerfettoSystemTracing)) {
+    // We have to ensure that we register all the data sources we care about.
+    TraceEventAgent::GetInstance();
     // To ensure System tracing connects we have to initialize the process wide
     // state. This Get() call ensures that the constructor has run.
     PerfettoTracedProcess::Get();