HTML Imports: Teach preloader about HTML Imports.

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 dc146e52c7777576225c3f6d8eb58bacd94bf2b7..fad50846d7c7547d5195e925f6ccdeb330ebbcfc 100644
--- a/Source/core/fetch/ResourceFetcher.cpp
+++ b/Source/core/fetch/ResourceFetcher.cpp
@@ -941,10 +941,10 @@ void ResourceFetcher::preload(Resource::Type type, FetchRequest& request, const
     if (type == Resource::MainResource)
         return;
 
-    ASSERT(type == Resource::Script || type == Resource::CSSStyleSheet || type == Resource::Image);
+    ASSERT(type == Resource::Script || type == Resource::CSSStyleSheet || type == Resource::Image || type == Resource::ImportResource);
 
     String encoding;
-    if (type == Resource::Script || type == Resource::CSSStyleSheet) {
+    if (type == Resource::Script || type == Resource::CSSStyleSheet || type == Resource::ImportResource) {
         encoding = charset.isEmpty() ? context().charset() : charset;
 
         // RequestContext for Resource::Image is set in fetchImage below.