WebXR: use fence status change time for timing

Using wallclock time for render completion can overestimate GPU usage if
the fence client wait is called on an already-completed fence.

Use the fence status change time instead to get a more accurate
measurement. It's technically best-effort, with fallback to wallclock
time, but in practice seems to be reliably available.

Bug: 1136276
Change-Id: I29c2830220de1dbac053cfeec8e4494f6d2eb42e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461959
Auto-Submit: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815241}

device/vr/android/arcore/arcore_gl.cc

@@ -37,6 +37,7 @@
 #include "ui/gl/android/surface_texture.h"
 #include "ui/gl/gl_bindings.h"
 #include "ui/gl/gl_context.h"
+#include "ui/gl/gl_fence_android_native_fence_sync.h"
 #include "ui/gl/gl_fence_egl.h"
 #include "ui/gl/gl_image_ahardwarebuffer.h"
 #include "ui/gl/gl_surface.h"
@@ -529,9 +530,20 @@ void ArCoreGl::GetRenderedFrameStats() {
   DCHECK(webxr_->HaveRenderingFrame());
   vr::WebXrFrame* frame = webxr_->GetRenderingFrame();
 
+  DCHECK(frame->render_completion_fence);
+  base::TimeTicks completion_time =
+      static_cast<gl::GLFenceAndroidNativeFenceSync*>(
+          frame->render_completion_fence.get())
+          ->GetStatusChangeTime();
+  if (completion_time.is_null()) {
+    // The fence status change time is best effort and may be unavailable.
+    // In that case, use wallclock time.
+    DVLOG(3) << __func__ << ": got null completion time, using wallclock";
+    completion_time = base::TimeTicks::Now();
+  }
+
   base::TimeDelta pose_to_submit = frame->time_js_submit - frame->time_pose;
-  base::TimeDelta submit_to_swap =
-      base::TimeTicks::Now() - frame->time_js_submit;
+  base::TimeDelta submit_to_swap = completion_time - frame->time_js_submit;
   TRACE_COUNTER2("gpu", "WebXR frame time (ms)", "javascript",
                  pose_to_submit.InMilliseconds(), "processing",
                  submit_to_swap.InMilliseconds());