Adds @UsedByReflection to the CronetUrlRequestContext constructor. This is...

Adds @UsedByReflection to the CronetUrlRequestContext constructor. This is necessary for UrlRequestContext to instantiate this class after proguarding.

BUG=

Review URL: https://codereview.chromium.org/1018213007

Cr-Commit-Position: refs/heads/master@{#322208}

components/cronet/android/java/src/org/chromium/net/CronetUrlRequestContext.java

@@ -15,6 +15,7 @@ import android.util.Log;
 import org.chromium.base.CalledByNative;
 import org.chromium.base.JNINamespace;
 import org.chromium.base.NativeClassQualifiedName;
+import org.chromium.base.UsedByReflection;
 
 import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -23,6 +24,7 @@ import java.util.concurrent.atomic.AtomicInteger;
  * UrlRequest context using Chromium HTTP stack implementation.
  */
 @JNINamespace("cronet")
+@UsedByReflection("UrlRequestContext.java")
 public class CronetUrlRequestContext extends UrlRequestContext  {
     private static final int LOG_NONE = 3;  // LOG(FATAL), no VLOG.
     private static final int LOG_DEBUG = -1;  // LOG(FATAL...INFO), VLOG(1)
@@ -39,6 +41,7 @@ public class CronetUrlRequestContext extends UrlRequestContext  {
     private long mUrlRequestContextAdapter = 0;
     private Thread mNetworkThread;
 
+    @UsedByReflection("UrlRequestContext.java")
     public CronetUrlRequestContext(Context context,
                                    UrlRequestContextConfig config) {
         CronetLibraryLoader.ensureInitialized(context, config);