[Web Payment][Android] Check for null pointers.

Before this patch, a null payment method section or unselected payment
instrument could result in a NullPointerException inside of the
isMicrotransactionUiApplicable() method in PaymentRequestImpl.

This patch null-checks the payment method section and the selected
payment instrument before accessing them.

After this patch, isMicrotransactionUiApplicable() will not throw
NullPointerException.

Bug: 1046938
Change-Id: I14335effa84c5ca44a2de960f63e25fe3eb5ee2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033676
Auto-Submit: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738331}

chrome/android/java/src/org/chromium/chrome/browser/payments/PaymentRequestImpl.java

@@ -1093,10 +1093,13 @@ public class PaymentRequestImpl
 
     /** @return Whether the microtransaction UI should be shown. */
     private boolean isMicrotransactionUiApplicable() {
-        if (!mIsUserGestureShow || mPaymentMethodsSection.getSize() != 1) return false;
+        if (!mIsUserGestureShow || mPaymentMethodsSection == null
+                || mPaymentMethodsSection.getSize() != 1) {
+            return false;
+        }
 
         PaymentInstrument instrument = (PaymentInstrument) mPaymentMethodsSection.getSelectedItem();
-        if (!instrument.isReadyForMicrotransaction()
+        if (instrument == null || !instrument.isReadyForMicrotransaction()
                 || TextUtils.isEmpty(instrument.accountBalance())) {
             return false;
         }