Add a Setting to make iframes load at VeryLow priority, disabled by default.

Source/core/fetch/ResourceFetcher.cpp

Index: Source/core/fetch/ResourceFetcher.cpp
diff --git a/Source/core/fetch/ResourceFetcher.cpp b/Source/core/fetch/ResourceFetcher.cpp
index 27f7aa1c4da11214ec03a1c2ae350bd25f784839..d02da3811d33b0d2f2d2a74f35d9d312e7e17f57 100644
--- a/Source/core/fetch/ResourceFetcher.cpp
+++ b/Source/core/fetch/ResourceFetcher.cpp
@@ -56,7 +56,7 @@ using blink::WebURLRequest;
 
 namespace blink {
 
-ResourceLoadPriority ResourceFetcher::loadPriority(Resource::Type type, const FetchRequest& request)
+ResourceLoadPriority ResourceFetcher::loadPriority(Resource::Type type, const FetchRequest& request, bool isLowPriorityIframe)
 {
     // TODO(yoav): Change it here so that priority can be changed even after it was resolved.
     if (request.priority() != ResourceLoadPriorityUnresolved)
@@ -64,7 +64,7 @@ ResourceLoadPriority ResourceFetcher::loadPriority(Resource::Type type, const Fe
 
     switch (type) {
     case Resource::MainResource:
-        return ResourceLoadPriorityVeryHigh;
+        return isLowPriorityIframe ? ResourceLoadPriorityVeryLow : ResourceLoadPriorityVeryHigh;
     case Resource::CSSStyleSheet:
         return ResourceLoadPriorityHigh;
     case Resource::Raw:
@@ -323,7 +323,7 @@ ResourcePtr<Resource> ResourceFetcher::requestResource(FetchRequest& request, co
         resource->setIdentifier(createUniqueIdentifier());
 
     if (!request.forPreload() || policy != Use) {
-        ResourceLoadPriority priority = loadPriority(factory.type(), request);
+        ResourceLoadPriority priority = loadPriority(factory.type(), request, context().isLowPriorityIframe());
         // When issuing another request for a resource that is already in-flight make
         // sure to not demote the priority of the in-flight request. If the new request
         // isn't at the same priority as the in-flight request, only allow promotions.