AW: mark SupportLibClient as final

This marks the SupportLibWebViewContentsClientAdapter as final. This
instance is 1:1 with WebViewContentsClientAdapter and is set during the
constructor, so it can be final.

Test: N/A
Change-Id: I288ea6784e3a6c613825f8256dcecf78a1201f2c
Reviewed-on: https://chromium-review.googlesource.com/1039156
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555308}

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

@@ -103,7 +103,7 @@ class WebViewContentsClientAdapter extends AwContentsClient {
     // The WebViewClient instance that was passed to WebView.setWebViewClient().
     protected WebViewClient mWebViewClient = sNullWebViewClient;
     // Some callbacks will be forwarded to this client for apps using the support library.
-    private SupportLibWebViewContentsClientAdapter mSupportLibClient;
+    private final SupportLibWebViewContentsClientAdapter mSupportLibClient;
     // The WebChromeClient instance that was passed to WebView.setContentViewClient().
     private WebChromeClient mWebChromeClient;
     // The listener receiving find-in-page API results.