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

Source/core/fetch/ResourceLoadPriorityOptimizer.cpp

Index: Source/core/fetch/ResourceLoadPriorityOptimizer.cpp
diff --git a/Source/core/fetch/ResourceLoadPriorityOptimizer.cpp b/Source/core/fetch/ResourceLoadPriorityOptimizer.cpp
index a95c0e1f8023162f5f8fd64b8100917a6fdcb88e..bdb63a71b1211cd01ef717cd71c68dc97b7e5731 100644
--- a/Source/core/fetch/ResourceLoadPriorityOptimizer.cpp
+++ b/Source/core/fetch/ResourceLoadPriorityOptimizer.cpp
@@ -101,7 +101,7 @@ void ResourceLoadPriorityOptimizer::updateImageResourcesWithLoadPriority()
             ResourceLoadPriorityLow : ResourceLoadPriorityVeryLow;
 
         if (priority != it->value->imageResource->resourceRequest().priority()) {
-            it->value->imageResource->resourceRequest().setPriority(priority, it->value->screenArea);
+            it->value->imageResource->mutableResourceRequest().setPriority(priority, it->value->screenArea);
             it->value->imageResource->didChangePriority(priority, it->value->screenArea);
         }
     }