Removing callers of blink::Platform::CreateDefaultURLLoaderFactory (2).

This CL removes callers of
blink::Platform::CreateDefaultURLLoaderFactory to help with eventual
removal of this virtual method from blink::Platform.

This CL fixes TestPluginLocalFrameClient.  It turns out that all tests
pass without overriding the CreateURLLoaderFactory that
TestPluginLocalFrameClient inherits from EmptyLocalFrameClient (and
which returns |nullptr|).

Bug: 891872
Change-Id: I419359754373055e3499ea0577a5eda41298a831
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250947Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779639}

third_party/blink/renderer/core/html/html_plugin_element_test.cc

@@ -25,10 +25,6 @@ class TestPluginLocalFrameClient : public EmptyLocalFrameClient {
   int plugin_created_count() const { return plugin_created_count_; }
 
  private:
-  std::unique_ptr<WebURLLoaderFactory> CreateURLLoaderFactory() override {
-    return Platform::Current()->CreateDefaultURLLoaderFactory();
-  }
-
   WebPluginContainerImpl* CreatePlugin(HTMLPlugInElement& element,
                                        const KURL& url,
                                        const Vector<String>& param_names,