[vr] Add a frame rate counter for the vr UI

chrome/browser/android/vr_shell/vr_shell_gl.cc

Index: chrome/browser/android/vr_shell/vr_shell_gl.cc
diff --git a/chrome/browser/android/vr_shell/vr_shell_gl.cc b/chrome/browser/android/vr_shell/vr_shell_gl.cc
index 2f6ca1858794f98ba5b079eec8914441c31f2b4f..1df330deafaa421af40c0bd45c7ad705bce6b629 100644
--- a/chrome/browser/android/vr_shell/vr_shell_gl.cc
+++ b/chrome/browser/android/vr_shell/vr_shell_gl.cc
@@ -13,6 +13,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/threading/thread_task_runner_handle.h"
+#include "cc/trees/frame_rate_counter.h"
 #include "chrome/browser/android/vr_shell/mailbox_to_surface_bridge.h"
 #include "chrome/browser/android/vr_shell/ui_elements.h"
 #include "chrome/browser/android/vr_shell/ui_interface.h"
@@ -84,6 +85,9 @@ static constexpr unsigned kPoseRingBufferSize = 8;
 // controller movement as a gesture.
 static constexpr float kMinAppButtonGestureAngleRad = 0.25;
 
+// UI has no "impl" (i.e., separate compositor) thread.
+static constexpr bool kHasImplThread = false;
+
 // Generate a quaternion representing the rotation from the negative Z axis
 // (0, 0, -1) to a specified vector. This is an optimized version of a more
 // general vector-to-vector calculation.
@@ -147,6 +151,7 @@ VrShellGl::VrShellGl(
       binding_(this),
       weak_vr_shell_(weak_vr_shell),
       main_thread_task_runner_(std::move(main_thread_task_runner)),
+      frame_rate_counter_(cc::FrameRateCounter::Create(kHasImplThread)),
       weak_ptr_factory_(this) {
   GvrInit(gvr_api);
 }
@@ -836,6 +841,15 @@ void VrShellGl::DrawFrame(int16_t frame_index) {
     TRACE_EVENT0("gpu", "VrShellGl::SwapBuffers");
     surface_->SwapBuffers();
   }
+
+  // This is an unused parameter when we do not have a compositor thread.
+  bool unused_is_a_software_compositor = false;
+
+  // After saving the timestamp, a reliable framerate should be available via
+  // GetAverageFPS().
+  // TODO(vollick): enable rendering of this framerate in a HUD.
+  frame_rate_counter_->SaveTimeStamp(current_time,

[mthiesse, 2017/04/07 15:36:33]

I know this is fairly lightweight, but we should probably only be generating
this data when using it, so putting it behind a debug flag for now might be
reasonable?

+                                     unused_is_a_software_compositor);

[mthiesse, 2017/04/07 15:36:33]

nit: Definitely personal preference, but I think this is overly verbose.
I would have written 
frame_rate_counter_->SaveTimeStamp(current_time, false /* unused */);
but feel free to ignore this.

 }
 
 void VrShellGl::DrawWorldElements(const gvr::Mat4f& head_pose) {