Make the startup pagefault count be gathered from a background task

Make Startup.BrowserMessageLoopStartHardFaultCount be gathered from a
TaskPriority::BACKGROUND task

Bug: 784853
Change-Id: Ia8772f9e66a20dabbd6e8fd7c73588bed421f9bc
Reviewed-on: https://chromium-review.googlesource.com/884377
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533912}

components/startup_metric_utils/browser/startup_metric_utils.cc

@@ -19,6 +19,8 @@
 #include "base/profiler/stack_sampling_profiler.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/sys_info.h"
+#include "base/task_scheduler/post_task.h"
+#include "base/task_scheduler/task_traits.h"
 #include "base/threading/platform_thread.h"
 #include "base/trace_event/trace_event.h"
 #include "build/build_config.h"
@@ -320,10 +322,10 @@ void RecordSystemUptimeHistogram() {
 }
 
 // On Windows, records the number of hard-faults that have occurred in the
-// current chrome.exe process since it was started. This is a nop on other
-// platforms.
-void RecordHardFaultHistogram() {
+// current chrome.exe process since it was started. This function uses an
+// expensive system call and should preferably be used in a background thread.
 #if defined(OS_WIN)
+void RecordHardFaultHistogram() {
   uint32_t hard_fault_count = 0;
 
   // Don't record histograms if unable to get the hard fault count.
@@ -374,8 +376,8 @@ void RecordHardFaultHistogram() {
         base::HistogramBase::kUmaTargetedHistogramFlag)
         ->Add(g_startup_temperature);
   }
-#endif  // defined(OS_WIN)
 }
+#endif  // defined(OS_WIN)
 
 // Converts a base::Time value to a base::TimeTicks value. The conversion isn't
 // exact, but by capturing Time::Now() as early as possible, the likelihood of a
@@ -568,7 +570,14 @@ void RecordBrowserMainMessageLoopStart(base::TimeTicks ticks,
   // histograms depend on it setting |g_startup_temperature| and
   // |g_startups_with_current_version|.
   RecordSameVersionStartupCount(pref_service);
-  RecordHardFaultHistogram();
+#if defined(OS_WIN)
+  // Record the hard page fault count in a background thread as this is quite
+  // expensive to compute.
+  base::PostTaskWithTraits(FROM_HERE,
+                           {base::TaskPriority::BACKGROUND,
+                            base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN},
+                           base::BindOnce(&RecordHardFaultHistogram));
+#endif
 
   // Record timing of the browser message-loop start time.
   base::StackSamplingProfiler::SetProcessMilestone(