Activate GPU main thread stack sampling through slow-reports

This CL is adding the support for the stack sampling profiler to the GPU
main thread. The |tracing_sampler_profiler| is an adaptor that forward
stack sample as trace events. These events are collected by slow-reports.

This code is only running when the trace category "cpu_sampler" is
activated and should not affect performance otherwise.

R=piman@chromium.org, ssid@chromium.org

Bug: 888580
Change-Id: Ie703d965c1d158581da6fac1ac0f6ea2157ed6f1
Reviewed-on: https://chromium-review.googlesource.com/1249534Reviewed-by: Siddhartha S <ssid@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594872}

content/gpu/gpu_main.cc

@@ -20,6 +20,7 @@
 #include "base/timer/hi_res_timer_manager.h"
 #include "base/trace_event/trace_event.h"
 #include "build/build_config.h"
+#include "components/tracing/common/tracing_sampler_profiler.h"
 #include "components/viz/service/main/viz_main_impl.h"
 #include "content/common/content_constants_internal.h"
 #include "content/common/content_switches_internal.h"
@@ -336,6 +337,12 @@ int GpuMain(const MainFunctionParams& parameters) {
 
   gpu_process.set_main_thread(child_thread);
 
+  // Setup tracing sampler profiler as early as possible.
+  auto tracing_sampler_profiler =
+      std::make_unique<tracing::TracingSamplerProfiler>(
+          base::PlatformThread::CurrentId());
+  tracing_sampler_profiler->OnMessageLoopStarted();
+
 #if defined(OS_ANDROID)
   base::trace_event::MemoryDumpManager::GetInstance()->RegisterDumpProvider(
       tracing::GraphicsMemoryDumpProvider::GetInstance(), "AndroidGraphics",