Update docs/webui_explainer.md to refer to std::make_unique.

Bug: 755727
Change-Id: I001537e46fa19ad521c1df58243fddbf4782e40b
Reviewed-on: https://chromium-review.googlesource.com/946123
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540558}

docs/webui_explainer.md

@@ -138,7 +138,7 @@ class DonutsUI : public content::WebUIController {
     content::WebUIDataSource::Add(source);
 
     // Handles messages from JavaScript to C++ via chrome.send().
-    web_ui->AddMessageHandler(base::MakeUnique<OvenHandler>());
+    web_ui->AddMessageHandler(std::make_unique<OvenHandler>());
   }
 };
 ```