[PaymentHandler] Comments for readability

Added comments:
* Comment why the ref over the weak-ref is chosen for WebContents.
* Comment why using an abbreviation.

Bug: 999196

Change-Id: I94eddbed050e131865869ac09521fa7275d48619
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1912912Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714834}

chrome/android/java/src/org/chromium/chrome/browser/payments/handler/PaymentHandlerMediator.java

@@ -23,7 +23,10 @@ import org.chromium.ui.modelutil.PropertyModel;
         extends WebContentsObserver implements BottomSheetObserver {
     private final PropertyModel mModel;
     private final Runnable mHider;
-    /** Postfixed with "Ref" to distinguish from mWebContent in WebContentsObserver. */
+    // Postfixed with "Ref" to distinguish from mWebContent in WebContentsObserver. Although
+    // referencing the same object, mWebContentsRef is preferable to WebContents here because
+    // mWebContents (a weak ref) requires null checks, while mWebContentsRef is guaranteed to be not
+    // null.
     private final WebContents mWebContentsRef;
 
     /**
@@ -37,6 +40,7 @@ import org.chromium.ui.modelutil.PropertyModel;
     /* package */ PaymentHandlerMediator(
             PropertyModel model, Runnable hider, WebContents webContents) {
         super(webContents);
+        assert webContents != null;
         mWebContentsRef = webContents;
         mModel = model;
         mHider = hider;

chrome/android/java/src/org/chromium/chrome/browser/payments/handler/toolbar/PaymentHandlerToolbarMediator.java

@@ -26,6 +26,7 @@ import java.net.URISyntaxException;
  * notifies the backend (the coordinator).
  */
 /* package */ class PaymentHandlerToolbarMediator extends WebContentsObserver {
+    // Abbreviated for the length limit.
     private static final String TAG = "PaymentHandlerTb";
     /** The delay (four video frames - for 60Hz) after which the hide progress will be hidden. */
     private static final long HIDE_PROGRESS_BAR_DELAY_MS = (1000 / 60) * 4;