[PaymentHandler] Set minimum load progress

Context: This is regarding the Bottom-sheet Payment Handler UI.
In slow connection, the toolbar on the UI appears empty and shows no
indicator of the current loading status, hardly distinguishing from
a state of freezing.

Old behavior: upon opening the PaymentHandlerUI, its toolbar shows
nothing but the payment app's url.

Change: set the minimum loading progress to be 5%. Apply it to the
initialization and the progress update.

New behavior: upon opening the PaymentHandlerUI, its toolbar shows
the payment app's url as well as a progress bar starting from at least
5%, which reassures users that the UI is busy loading the page.

Bug: 1029815
Change-Id: I05147c15e9c732c75a039c9c20fd021382ec8a72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1951649Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722498}

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

@@ -48,7 +48,8 @@ public class PaymentHandlerToolbarCoordinator {
         mToolbarView = new PaymentHandlerToolbarView(context, observer);
         PropertyModel model = new PropertyModel.Builder(PaymentHandlerToolbarProperties.ALL_KEYS)
                                       .with(PaymentHandlerToolbarProperties.PROGRESS_VISIBLE, true)
-                                      .with(PaymentHandlerToolbarProperties.LOAD_PROGRESS, 0)
+                                      .with(PaymentHandlerToolbarProperties.LOAD_PROGRESS,
+                                              PaymentHandlerToolbarMediator.MINIMUM_LOAD_PROGRESS)
                                       .with(PaymentHandlerToolbarProperties.SECURITY_ICON,
                                               ConnectionSecurityLevel.NONE)
                                       .build();

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

@@ -30,6 +30,11 @@ import java.net.URISyntaxException;
     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;
+    /**
+     * The minimum load progress that can be shown when a page is loading.  This is not 0 so that
+     * it's obvious to the user that something is attempting to load.
+     */
+    /* package */ static final float MINIMUM_LOAD_PROGRESS = 0.05f;
 
     private final PropertyModel mModel;
     private final PaymentHandlerToolbarObserver mObserver;
@@ -102,6 +107,7 @@ import java.net.URISyntaxException;
 
     @Override
     public void loadProgressChanged(float progress) {
+        assert progress <= 1.0;
         if (progress == 1.0) return;
         // If the load restarts when the progress bar is waiting to hide, cancel the handler
         // callbacks.
@@ -110,7 +116,8 @@ import java.net.URISyntaxException;
             mHideProgressBarHandler = null;
         }
         mModel.set(PaymentHandlerToolbarProperties.PROGRESS_VISIBLE, true);
-        mModel.set(PaymentHandlerToolbarProperties.LOAD_PROGRESS, progress);
+        mModel.set(PaymentHandlerToolbarProperties.LOAD_PROGRESS,
+                Math.max(progress, MINIMUM_LOAD_PROGRESS));
     }
 
     @DrawableRes