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());

[pauljensen, 2015/12/29 16:35:53]

I'm worried about this automatic silent fallback.  I dislike silent fallback
because embedders might try out Cronet and think it doesn't perform any better
than their existing HttpURLConnection code, but they could silently be using the
HttpURLConnection impl of Cronet.  Perhaps we should use the legacyMode()
plumbing we already have to control this fallback?  This code could be something
like:

if (builder.legacyMode()) {
  cronetEngine = new JavaCronetEngine(builder.getUserAgent());
} else {
  cronetEngine = createCronetEngine(builder);
}

[Charles, 2016/01/05 21:53:56]

The problem with that is that you now need a code change in order to use the
java impl - this means that you have to have two different source files when
building, rather than making the decision based on the structure of the build
graph alone. If you want, I could add a log statement that provides an explicit
warning about the fact that we're loading the legacy version of the stack. Would
assuage your concerns?

         }
         Log.i(TAG, "Using network stack: " + cronetEngine.getVersionString());
         return cronetEngine;