WebPrerenderingSupport: tiny style nits

Bug: None
Change-Id: I7c1fb0026eb6db686b79dac055d9d792a82ebf13
Reviewed-on: https://chromium-review.googlesource.com/1104473
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568347}

third_party/blink/renderer/platform/exported/web_prerendering_support.cc

@@ -34,14 +34,17 @@ namespace blink {
 
 WebPrerenderingSupport* WebPrerenderingSupport::platform_ = nullptr;
 
+// static
 void WebPrerenderingSupport::Initialize(WebPrerenderingSupport* platform) {
   platform_ = platform;
 }
 
+// static
 void WebPrerenderingSupport::Shutdown() {
   platform_ = nullptr;
 }
 
+// static
 WebPrerenderingSupport* WebPrerenderingSupport::Current() {
   return platform_;
 }