Add FLAG_IMMUTABLE to PendingIntent in FRE

The PendingIntent created by FRE is not expected to be changed after
creation.

Bug: 1164088
Change-Id: I8330329bb7f340c6375a82db23f11e2880894abd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617120Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Wenyu Fu <wenyufu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841331}

chrome/android/java/src/org/chromium/chrome/browser/firstrun/FreIntentCreator.java

@@ -9,6 +9,7 @@ import android.app.PendingIntent;
 import android.content.Context;
 import android.content.Intent;
 import android.content.res.TypedArray;
+import android.os.Build;
 import android.os.Bundle;
 import android.os.SystemClock;
 import android.text.TextUtils;
@@ -142,6 +143,9 @@ public class FreIntentCreator {
             Intent intentToLaunchAfterFreComplete, boolean requiresBroadcast) {
         final PendingIntent pendingIntent;
         int pendingIntentFlags = PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_ONE_SHOT;
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
+            pendingIntentFlags |= PendingIntent.FLAG_IMMUTABLE;
+        }
         if (requiresBroadcast) {
             pendingIntent = PendingIntent.getBroadcast(
                     context, 0, intentToLaunchAfterFreComplete, pendingIntentFlags);