[NoStatePrefetch] Support only GET and HEAD

chrome/browser/prerender/prerender_resource_throttle.cc

Index: chrome/browser/prerender/prerender_resource_throttle.cc
diff --git a/chrome/browser/prerender/prerender_resource_throttle.cc b/chrome/browser/prerender/prerender_resource_throttle.cc
index 200e2865557792db5a3c5c694c05a6bfac76474b..0f7ca5225376443b35c04edd3ba8cd2921ee9f9a 100644
--- a/chrome/browser/prerender/prerender_resource_throttle.cc
+++ b/chrome/browser/prerender/prerender_resource_throttle.cc
@@ -121,7 +121,7 @@ void PrerenderResourceThrottle::WillStartRequestOnUI(
   if (prerender_contents) {
     // Abort any prerenders that spawn requests that use unsupported HTTP
     // methods or schemes.
-    if (!PrerenderManager::IsValidHttpMethod(method)) {
+    if (!prerender_contents->IsValidHttpMethod(method)) {

[mmenke, 2016/09/20 13:31:18]

Would it make more sense just to cancel the individual request, not the entire
prerender, when this fails for prefetch prerenders?  For "real" prerenders, if
we cancel a request, the entire prerender is unusable.  For prefetch prerenders,
however, the whole point is the resources we fetch, so there doesn't seem to be
a reason to cancel the prerender other resources, unless this is the main frame
request, right?

[droger, 2016/09/20 15:07:27]

I think you are right indeed, we could cancel the individual request instead of
the whole prefetch.

To do this, it would probably be enough to set cancel to true, but only call
>Destroy() if the resource_type is "main frame". I will play with this.

[droger, 2016/09/21 09:57:48]

Done, although the scenario where a prefetch subresource has an invalid method
cannot actually happen currently (since these requests are created by the HTML
preload scanner).

[mmenke, 2016/09/21 16:56:20]

Good point, though I suppose extensions and Chrome components can do all sorts
of weird things.  May make sense just to remove this class entirely, once we get
rid of prerenders.

       prerender_contents->Destroy(FINAL_STATUS_INVALID_HTTP_METHOD);
       cancel = true;
     } else if (!PrerenderManager::DoesSubresourceURLHaveValidScheme(url)) {