Make stack sampling profiler sample beyond startup.

base/profiler/stack_sampling_profiler.h

Index: base/profiler/stack_sampling_profiler.h
diff --git a/base/profiler/stack_sampling_profiler.h b/base/profiler/stack_sampling_profiler.h
index a5e30e9fa0e892c0c588be2ea1ce1f144d2c741c..16481bce7fd22617f16b6d5d4efe4f4e23e81176 100644
--- a/base/profiler/stack_sampling_profiler.h
+++ b/base/profiler/stack_sampling_profiler.h
@@ -16,6 +16,7 @@
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/macros.h"
+#include "base/optional.h"
 #include "base/strings/string16.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/platform_thread.h"
@@ -215,13 +216,18 @@ class BASE_EXPORT StackSamplingProfiler {
   // are move-only. Other threads, including the UI thread, may block on
   // callback completion so this should run as quickly as possible.
   //
+  // After collection completion, the callback may instruct the profiler to do
+  // additional collection(s) by returning a SamplingParams object to indicate
+  // collection should be started again.
+  //
   // IMPORTANT NOTE: The callback is invoked on a thread the profiler
   // constructs, rather than on the thread used to construct the profiler and
   // set the callback, and thus the callback must be callable on any thread. For
   // threads with message loops that create StackSamplingProfilers, posting a
   // task to the message loop with the moved (i.e. std::move) profiles is the
   // thread-safe callback implementation.
-  using CompletedCallback = Callback<void(CallStackProfiles)>;
+  using CompletedCallback =
+      Callback<Optional<SamplingParams>(CallStackProfiles)>;
 
   // Creates a profiler for the CURRENT thread that sends completed profiles
   // to |callback|. An optional |test_delegate| can be supplied by tests.