[animation-worklet] Add comment re potential precision loss in time conversion

Bug: 827194
Change-Id: Ib21f0c55e2a196f75f3aa27ee0d0bc997a5b4bd1
Reviewed-on: https://chromium-review.googlesource.com/986750
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549860}

third_party/blink/renderer/modules/animationworklet/effect_proxy.h

@@ -19,8 +19,17 @@ class MODULES_EXPORT EffectProxy : public ScriptWrappable {
  public:
   EffectProxy() = default;
 
-  void setLocalTime(double time) {
-    local_time_ = WTF::TimeDelta::FromMillisecondsD(time);
+  void setLocalTime(double time_ms) {
+    // Convert double to TimeDelta because cc/animation expects TimeDelta.
+    //
+    // Note on precision loss: TimeDelta has microseconds precision which is
+    // also the precision recommended by the web animation specification as well
+    // [1]. If the input time value has a bigger precision then the conversion
+    // causes precision loss. Doing the conversion here ensures that reading the
+    // value back provides the actual value we use in further computation which
+    // is the least surprising path.
+    // [1] https://drafts.csswg.org/web-animations/#precision-of-time-values
+    local_time_ = WTF::TimeDelta::FromMillisecondsD(time_ms);
   }
 
   double localTime() const { return local_time_.InMillisecondsF(); }