Add HttpUrlConnection backed implementation of CronetEngine.

components/cronet/android/api/src/org/chromium/net/CronetEngine.java

Index: components/cronet/android/api/src/org/chromium/net/CronetEngine.java
diff --git a/components/cronet/android/api/src/org/chromium/net/CronetEngine.java b/components/cronet/android/api/src/org/chromium/net/CronetEngine.java
index be497add40d0af446fe4e2eca1b978a8ad3fcdae..d6a953824ddd7ef0c660cb15a1a580eefb524d31 100644
--- a/components/cronet/android/api/src/org/chromium/net/CronetEngine.java
+++ b/components/cronet/android/api/src/org/chromium/net/CronetEngine.java
@@ -9,6 +9,7 @@ import android.support.annotation.IntDef;
 import android.util.Log;
 
 import java.io.File;
+import java.io.IOException;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.reflect.Constructor;
@@ -568,8 +569,10 @@ public abstract class CronetEngine {
      * @deprecated Use {@link UrlRequest.Builder#build}.
      */
     @Deprecated
-    public abstract UrlRequest createRequest(
-            String url, UrlRequest.Callback callback, Executor executor);
+    public final UrlRequest createRequest(
+            String url, UrlRequest.Callback callback, Executor executor) {
+        return createRequest(url, callback, executor, UrlRequest.Builder.REQUEST_PRIORITY_MEDIUM);
+    }
 
     /**
      * Creates a {@link UrlRequest} object. All callbacks will
@@ -760,7 +763,7 @@ public abstract class CronetEngine {
      * @param url URL of resource to connect to.
      * @return an {@link java.net.HttpURLConnection} instance implemented by this CronetEngine.
      */
-    public abstract URLConnection openConnection(URL url);
+    public abstract URLConnection openConnection(URL url) throws IOException;
 
     /**
      * Establishes a new connection to the resource specified by the {@link URL} {@code url}
@@ -777,7 +780,8 @@ public abstract class CronetEngine {
      *         javadocs are built with nodeprecated="yes".
      *         TODO(pauljensen): Expose once implemented, http://crbug.com/418111
      */
-    @SuppressWarnings("DepAnn") public abstract URLConnection openConnection(URL url, Proxy proxy);
+    @SuppressWarnings("DepAnn")
+    public abstract URLConnection openConnection(URL url, Proxy proxy) throws IOException;
 
     /**
      * Creates a {@link URLStreamHandlerFactory} to handle HTTP and HTTPS
@@ -830,9 +834,7 @@ public abstract class CronetEngine {
             cronetEngine = createCronetEngine(builder);
         }
         if (cronetEngine == null) {
-            // TODO(mef): Fallback to stub implementation. Once stub
-            // implementation is available merge with createCronetFactory.
-            cronetEngine = createCronetEngine(builder);
+            cronetEngine = new JavaCronetEngine(builder.getUserAgent());
         }
         Log.i(TAG, "Using network stack: " + cronetEngine.getVersionString());
         return cronetEngine;