[PaymentHandler] Don't close UI on iframe loading fails

Context:
Having iframe navigating multiple times would make the payment handler
WebContents invokes the "didFailLoad" callback. Currently, didFailLoad
applies for both the mainframe and subframes. The UI shouldn't close
when the load failure happens on subframes.

Before:
The UI closes when the subframes fail to load.

After:
The UI doesn't close when the subframes fail to load.

Bug: 1137929

Change-Id: Iaba361e895d6bb4e46b0fb969a87958e9daebe57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2469536Reviewed-by: Sahel Sharify <sahel@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818316}

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

@@ -270,6 +270,7 @@ import java.lang.annotation.RetentionPolicy;
     // Implement WebContentsObserver:
     @Override
     public void didFailLoad(boolean isMainFrame, int errorCode, String failingUrl) {
+        if (!isMainFrame) return;
         mHandler.post(() -> {
             mCloseReason = CloseReason.FAIL_LOAD;
             mHider.run();