Allow top-level navigation in extension pop-ups if it only triggers a download.

chrome/renderer/chrome_content_renderer_client.cc

Index: chrome/renderer/chrome_content_renderer_client.cc
diff --git a/chrome/renderer/chrome_content_renderer_client.cc b/chrome/renderer/chrome_content_renderer_client.cc
index e697090a73f8365f19011e067d2accc10792fcad..de14cb2d7570fd28d69d16a819ee8928eff23d19 100644
--- a/chrome/renderer/chrome_content_renderer_client.cc
+++ b/chrome/renderer/chrome_content_renderer_client.cc
@@ -1096,6 +1096,16 @@ bool ChromeContentRendererClient::ShouldFork(WebLocalFrame* frame,
     return true;
   }
 
+  // TODO(lukasza): https://crbug.com/650694: For now, we skip the rest for POST
+  // submissions.  This is because 1) in M54 there are some remaining issues
+  // with POST in OpenURL path (e.g. https://crbug.com/648648) and 2) OpenURL
+  // path regresses (blocks) navigations that result in downloads
+  // (https://crbug.com/646261).  In the long-term we should avoid forking for
+  // extensions (not hosted apps though) altogether and rely on transfers logic
+  // instead.
+  if (http_method != "GET")
+    return false;
+
   // If |url| matches one of the prerendered URLs, stop this navigation and try
   // to swap in the prerendered page on the browser process. If the prerendered
   // page no longer exists by the time the OpenURL IPC is handled, a normal