[Payments] Add extra comments to explain implicit usage of the animation duration

chrome/android/java/src/org/chromium/chrome/browser/payments/ui/PaymentRequestUI.java

Index: chrome/android/java/src/org/chromium/chrome/browser/payments/ui/PaymentRequestUI.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/payments/ui/PaymentRequestUI.java b/chrome/android/java/src/org/chromium/chrome/browser/payments/ui/PaymentRequestUI.java
index 5bdcbc66827fc53812053cf70a775b706c83e016..b2273b0bd30c019f47406afad33e3e02888292ac 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/payments/ui/PaymentRequestUI.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/payments/ui/PaymentRequestUI.java
@@ -284,7 +284,12 @@ public class PaymentRequestUI implements DialogInterface.OnDismissListener, View
         }
     }
 
-    /** Length of the animation to either show the UI or expand it to full height. */
+    /**
+     * Length of the animation to either show the UI or expand it to full height.
+     * Note that click of 'Pay' button is not accepted until the animation is done, so this duration
+     * also serves the function of preventing the user from accidently double-clicking on the screen
+     * when triggering payment and thus authorizing unwanted transaction.
+     */
     private static final int DIALOG_ENTER_ANIMATION_MS = 225;
 
     /** Length of the animation to hide the bottom sheet UI. */