WebApk needs to be restarted to notice that host browser was uninstalled.

chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java

Index: chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java
diff --git a/chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java b/chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java
index 693cb384e5b99553502f4d6c03a657d502e3e931..0331d40d759c42bea52172c89c1a447cca0a332b 100644
--- a/chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java
+++ b/chrome/android/webapk/shell_apk/src/org/chromium/webapk/shell_apk/MainActivity.java
@@ -9,6 +9,7 @@ import android.content.ActivityNotFoundException;
 import android.content.Context;
 import android.content.Intent;
 import android.content.SharedPreferences;
+import android.content.pm.PackageManager;
 import android.content.pm.ResolveInfo;
 import android.net.Uri;
 import android.os.AsyncTask;
@@ -98,13 +99,14 @@ public class MainActivity extends Activity {
 
         String runtimeHostInPreferences = WebApkUtils.getHostBrowserFromSharedPreference(this);
         String runtimeHost = WebApkUtils.getHostBrowserPackageName(this);
+        boolean isInstalled = isInstalled(getPackageManager(), runtimeHost);
         if (!TextUtils.isEmpty(runtimeHostInPreferences)
-                && !runtimeHostInPreferences.equals(runtimeHost)) {
+                && (!runtimeHostInPreferences.equals(runtimeHost) || !isInstalled)) {
             deleteSharedPref(this);
             deleteInternalStorageAsync();
         }
 
-        if (!TextUtils.isEmpty(runtimeHost)) {
+        if (isInstalled) {
             launchInHostBrowser(runtimeHost);
             finish();
             return;
@@ -275,4 +277,16 @@ public class MainActivity extends Activity {
         InstallHostBrowserDialog.show(this, listener, hostUrl, lastResortHostBrowserPackageName,
                 lastResortHostBrowserApplicationName, R.drawable.last_resort_runtime_host_logo);
     }
+
+    /** Returns whether the application is installed. */
+    private static boolean isInstalled(PackageManager packageManager, String packageName) {
+        if (TextUtils.isEmpty(packageName)) return false;
+
+        try {
+            packageManager.getApplicationInfo(packageName, 0);
+        } catch (PackageManager.NameNotFoundException e) {
+            return false;
+        }
+        return true;
+    }
 }