Stop attaching 'cache-control: max-age=0' in sendBeacon

We've attached the header to requests initiated by sendBeacon() but it
is not spec conformant. Let's stop doing that.

Bug: 755725
Change-Id: Ia8b85a27b84c77b35edc211c325490467c853330
Reviewed-on: https://chromium-review.googlesource.com/952104Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541719}

third_party/WebKit/Source/core/loader/PingLoader.cpp

@@ -187,7 +187,6 @@ bool SendBeaconCommon(LocalFrame* frame,
 
   ResourceRequest request(url);
   request.SetHTTPMethod(HTTPNames::POST);
-  request.SetHTTPHeaderField(HTTPNames::Cache_Control, "max-age=0");
   request.SetKeepalive(true);
   request.SetRequestContext(WebURLRequest::kRequestContextBeacon);
   beacon.Serialize(request);