Save As for content retrieved via POST works in most circumstances.

content/browser/download/download_manager_impl.cc

Index: content/browser/download/download_manager_impl.cc
diff --git a/content/browser/download/download_manager_impl.cc b/content/browser/download/download_manager_impl.cc
index 65f60c9c1c8162aae0d4f785538ad6dab76c0c6c..3cd4360f50b85316a09880585127d5e861010290 100644
--- a/content/browser/download/download_manager_impl.cc
+++ b/content/browser/download/download_manager_impl.cc
@@ -36,6 +36,7 @@
 #include "content/public/browser/notification_types.h"
 #include "content/public/browser/render_process_host.h"
 #include "content/public/browser/web_contents_delegate.h"
+#include "net/base/upload_data.h"
 
 // TODO(benjhayden): Change this to DCHECK when we have more debugging
 // information from the next dev cycle, before the next stable/beta branch is
@@ -56,10 +57,11 @@ namespace {
 
 // Param structs exist because base::Bind can only handle 6 args.
 struct URLParams {
-  URLParams(const GURL& url, const GURL& referrer)
-    : url_(url), referrer_(referrer) {}
+  URLParams(const GURL& url, const GURL& referrer, int64 post_id)
+    : url_(url), referrer_(referrer), post_id_(post_id) {}
   GURL url_;
   GURL referrer_;
+  int64 post_id_;
 };
 
 struct RenderParams {
@@ -78,6 +80,15 @@ void BeginDownload(const URLParams& url_params,
   scoped_ptr<net::URLRequest> request(
       new net::URLRequest(url_params.url_, resource_dispatcher_host));
   request->set_referrer(url_params.referrer_.spec());
+  if (url_params.post_id_ >= 0) {

[Randy Smith (Not in Mondays), 2012/02/02 19:46:10]

If we decide that we eventually want to support POST with bodies that won't hit
in cache (see related comment), there should be a TODO comment here.

[cbentzel, 2012/02/03 15:54:47]

Done.

+    // The POST in this case does not have an actual body, and only works
+    // when retrieving data from cache.
+    DCHECK(prefer_cache);
+    request->set_method("POST");
+    scoped_refptr<net::UploadData> upload_data = new net::UploadData();
+    upload_data->set_identifier(url_params.post_id_);
+    request->set_upload(upload_data);
+  }
   resource_dispatcher_host->BeginDownload(
       request.Pass(), prefer_cache, save_info,
       DownloadResourceHandler::OnStartedCallback(),
@@ -835,6 +846,7 @@ void DownloadManagerImpl::DownloadUrl(
     const GURL& referrer,
     const std::string& referrer_charset,
     bool prefer_cache,
+    int64 post_id,
     const DownloadSaveInfo& save_info,
     WebContents* web_contents) {
   ResourceDispatcherHost* resource_dispatcher_host =
@@ -847,7 +859,7 @@ void DownloadManagerImpl::DownloadUrl(
       BrowserThread::IO, FROM_HERE,
       base::Bind(
           &BeginDownload,
-          URLParams(url, referrer),
+          URLParams(url, referrer, post_id),
           prefer_cache,
           save_info,
           resource_dispatcher_host,