Don't attempt to revalidate a Resource with a 304 response.

third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp

Index: third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp
diff --git a/third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp b/third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp
index 1e493de46d724485056e2a6dce127cda6eecf75c..2eb9236cd3e895ae26276521501002182bdcb589 100644
--- a/third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp
+++ b/third_party/WebKit/Source/core/fetch/ResourceFetcher.cpp
@@ -705,9 +705,16 @@ ResourceFetcher::RevalidationPolicy ResourceFetcher::determineRevalidationPolicy
         return Reload;
 
     // Certain requests (e.g., XHRs) might have manually set headers that require revalidation.
-    // FIXME: In theory, this should be a Revalidate case. In practice, the MemoryCache revalidation path assumes a whole bunch
+    // In theory, this should be a Revalidate case. In practice, the MemoryCache revalidation path assumes a whole bunch
     // of things about how revalidation works that manual headers violate, so punt to Reload instead.
-    if (request.isConditional())
+    //
+    // Similarly, a request with manually added revalidation headers can lead
+    // to a 304 response for a request that wasn't flagged as a revalidation
+    // attempt. Normally, successful revalidation will maintain the original
+    // response's status code, but for a manual revalidation the response code
+    // aremains 304. In this case, the Resource likely has insufficient context

[hiroshige, 2016/09/23 16:00:19]

nit: s/aremains/remains/?

[Nate Chapin, 2016/09/23 20:45:51]

Done.

+    // to provide a useful revalidation.

[hiroshige, 2016/09/23 16:00:19]

This looks like this condition is only for preventing Revalidation, but actually
also we should prevent Use.

[hiroshige, 2016/09/23 16:00:19]

Could you refer to the crbug entry to provide more context?

[Nate Chapin, 2016/09/23 20:45:50]

Done.

[Nate Chapin, 2016/09/23 20:45:50]

Done.

+    if (request.isConditional() || existingResource->response().httpStatusCode() == 304)
         return Reload;
 
     // Don't try to reuse an in-progress async request for a new sync request.