Add IO Thread to TestBlinkWebUnitTestSupport

This is required to support moving code from content/renderer
to Blink (i.e., Onion Souping) that uses the IO Thread.

Bug: 928021
Change-Id: I67210ee7e958403d559c2fc99783699991e7ad92
Reviewed-on: https://chromium-review.googlesource.com/c/1450266Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628915}

content/test/test_blink_web_unit_test_support.cc

@@ -18,6 +18,7 @@
 #include "build/build_config.h"
 #include "cc/trees/layer_tree_settings.h"
 #include "content/app/mojo/mojo_init.h"
+#include "content/child/child_process.h"
 #include "content/public/common/service_names.mojom.h"
 #include "content/renderer/loader/web_data_consumer_handle_impl.h"
 #include "content/renderer/loader/web_url_loader_impl.h"
@@ -288,6 +289,12 @@ blink::WebString TestBlinkWebUnitTestSupport::DefaultLocale() {
   return blink::WebString::FromASCII("en-US");
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+TestBlinkWebUnitTestSupport::GetIOTaskRunner() const {
+  return ChildProcess::current() ? ChildProcess::current()->io_task_runner()
+                                 : nullptr;
+}
+
 blink::WebURLLoaderMockFactory*
 TestBlinkWebUnitTestSupport::GetURLLoaderMockFactory() {
   return url_loader_factory_.get();

content/test/test_blink_web_unit_test_support.h

@@ -49,6 +49,7 @@ class TestBlinkWebUnitTestSupport : public BlinkPlatformImpl {
       const blink::WebString& value1,
       const blink::WebString& value2) override;
   blink::WebString DefaultLocale() override;
+  scoped_refptr<base::SingleThreadTaskRunner> GetIOTaskRunner() const override;
 
   blink::WebURLLoaderMockFactory* GetURLLoaderMockFactory() override;