[Cronet] Add callback wrapper classes to enforce API version checking.

components/cronet/android/java/src/org/chromium/net/impl/CronetEngineBuilderImpl.java

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 package org.chromium.net.impl;
 
 import android.content.Context;
 import android.support.annotation.IntDef;
 import android.support.annotation.VisibleForTesting;
 import android.util.Log;
 
@@ skipping 69 matching lines @@
 
     // Private fields are simply storage of configuration for the resulting CronetEngine.
     // See setters below for verbose descriptions.
     private final Context mApplicationContext;
     private final List<QuicHint> mQuicHints = new LinkedList<>();
     private final List<Pkp> mPkps = new LinkedList<>();
     private boolean mPublicKeyPinningBypassForLocalTrustAnchorsEnabled;
     private String mUserAgent;
     private String mStoragePath;
     private boolean mLegacyModeEnabled;
-    private CronetEngine.Builder.LibraryLoader mLibraryLoader;
+    private VersionSafeCallbacks.LibraryLoader mLibraryLoader;
     private boolean mQuicEnabled;
     private boolean mHttp2Enabled;
     private boolean mSdchEnabled;
     private String mDataReductionProxyKey;
     private String mDataReductionProxyPrimaryProxy;
     private String mDataReductionProxyFallbackProxy;
     private String mDataReductionProxySecureProxyCheckUrl;
     private boolean mDisableCache;
     private int mHttpCacheMode;
     private long mHttpCacheMaxSize;
@@ skipping 50 matching lines @@
         mLegacyModeEnabled = value;
         return this;
     }
 
     private boolean legacyMode() {
         return mLegacyModeEnabled;
     }
 
     @Override
     public CronetEngineBuilderImpl setLibraryLoader(CronetEngine.Builder.LibraryLoader loader) {
-        mLibraryLoader = loader;
+        mLibraryLoader = new VersionSafeCallbacks.LibraryLoader(loader);
         return this;
     }
 
-    CronetEngine.Builder.LibraryLoader libraryLoader() {
+    VersionSafeCallbacks.LibraryLoader libraryLoader() {
         return mLibraryLoader;
     }
 
     @Override
     public CronetEngineBuilderImpl enableQuic(boolean value) {
         mQuicEnabled = value;
         return this;
     }
 
     boolean quicEnabled() {
@@ skipping 315 matching lines @@
         } catch (ClassNotFoundException e) {
             // Leave as null.
             Log.i(TAG, "Class loader " + loader + " cannot find Cronet engine implementation: "
                             + name + ". Will try to find an alternative implementation.");
         } catch (Exception e) {
             throw new IllegalStateException("Cannot instantiate: " + name, e);
         }
         return cronetEngine;
     }
 }