Fix the drp crash for real.

There are 2 different path that can possibly lead to
SetDataReductionProxyEnalbed:
1. AwContents initialization -> InitDataReductionProxyIfNecessary path
2. AwContents statistics SetDataReductionProxyEnalbed JNI path

In both paths, we need to make sure before SetDataReductionProxyEnabed
is called, we have set the statistics object on
DataReductionProxySettings.

BUG=b/17998706

Review URL: https://codereview.chromium.org/671823002

Cr-Commit-Position: refs/heads/master@{#300730}

android_webview/browser/aw_browser_context.cc

@@ -217,8 +217,7 @@ void AwBrowserContext::CreateUserPrefServiceIfNecessary() {
         user_pref_service_.get(),
         GetRequestContext());
 
-    data_reduction_proxy_settings_->SetDataReductionProxyEnabled(
-        data_reduction_proxy_enabled_);
+    SetDataReductionProxyEnabled(data_reduction_proxy_enabled_);
   }
 }