VR: Fix rounding error in VSync computation.

Also fixes the timebase being off by a factor of 1000, though this
didn't actually cause issues...

Bug: 745866
Change-Id: Ia29646ce935bf66b45a5f6aee10bf7715a3fea17
Reviewed-on: https://chromium-review.googlesource.com/575700
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487648}

chrome/browser/android/vr_shell/vr_shell_delegate.cc

@@ -117,7 +117,7 @@ void VrShellDelegate::UpdateVSyncInterval(JNIEnv* env,
                                           jlong timebase_nanos,
                                           jlong interval_micros) {
   vsync_timebase_ = base::TimeTicks() +
-                    base::TimeDelta::FromMilliseconds(timebase_nanos / 1000);
+                    base::TimeDelta::FromMicroseconds(timebase_nanos / 1000);
   vsync_interval_ = base::TimeDelta::FromMicroseconds(interval_micros);
   if (gvr_delegate_) {
     gvr_delegate_->UpdateVSyncInterval(vsync_timebase_, vsync_interval_);

chrome/browser/android/vr_shell/vr_shell_gl.cc

@@ -1067,18 +1067,20 @@ void VrShellGl::OnVSync() {
     return;
 
   base::TimeTicks now = base::TimeTicks::Now();
-  base::TimeTicks target;
-  target = now + vsync_interval_;
-  int64_t intervals = (target - vsync_timebase_) / vsync_interval_;
+  base::TimeTicks target = now + vsync_interval_;
+  double intervalsF =
+      (target - vsync_timebase_).InSecondsF() / vsync_interval_.InSecondsF();
+  uint64_t intervals = std::llround(intervalsF);
   target = vsync_timebase_ + intervals * vsync_interval_;
   task_runner_->PostDelayedTask(FROM_HERE, vsync_task_.callback(),
                                 target - now);
-  base::TimeDelta current = target - vsync_interval_ - base::TimeTicks();
+
+  base::TimeDelta current_time = target - vsync_interval_ - base::TimeTicks();
   if (!callback_.is_null()) {
-    SendVSync(current, base::ResetAndReturn(&callback_));
+    SendVSync(current_time, base::ResetAndReturn(&callback_));
   } else {
     pending_vsync_ = true;
-    pending_time_ = current;
+    pending_time_ = current_time;
   }
   if (!ShouldDrawWebVr()) {
     DrawFrame(-1);