modify getWebViewClient behavior

getWebViewClient returns the default client instead of null when the client is
not yet set.

BUG=627248

Implement getWebViewClient and getWebChromeClient

Implement getWebViewClient and getWebChromeClient for new APIs in Android

Review-Url: https://codereview.chromium.org/2384063003
Cr-Commit-Position: refs/heads/master@{#422547}

android_webview/glue/java/src/com/android/webview/chromium/WebViewContentsClientAdapter.java

@@ -172,8 +172,7 @@ public class WebViewContentsClientAdapter extends AwContentsClient {
     }
 
     WebViewClient getWebViewClient() {
-        // If this hasn't been set yet, return null instead of exposing sNullWebViewClient
-        return mWebViewClient == sNullWebViewClient ? null : mWebViewClient;
+        return mWebViewClient;
     }
 
     void setWebChromeClient(WebChromeClient client) {