Split out const/non-const variants of Resource::resourceRequest().

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 317c51716bbcc49523e575de5e26b6843e25d916..6f5a53d02b8fb25e475ea13b7ef4348872064ae3 100644
--- a/Source/core/fetch/ResourceFetcher.cpp
+++ b/Source/core/fetch/ResourceFetcher.cpp
@@ -751,7 +751,7 @@ ResourcePtr<Resource> ResourceFetcher::requestResource(Resource::Type type, Fetc
     if (!request.forPreload() || policy != Use) {
         ResourceLoadPriority priority = loadPriority(type, request);
         if (priority != resource->resourceRequest().priority()) {
-            resource->resourceRequest().setPriority(priority);
+            resource->mutableResourceRequest().setPriority(priority);
             resource->didChangePriority(priority, 0);
         }
     }