Add hint_preload_subresources as a way of enabling preload hinting.

net/instaweb/rewriter/public/rewrite_options.h

@@ -147,6 +147,7 @@ class RewriteOptions {
     kFlattenCssImports,
     kFlushSubresources,
     kHandleNoscriptRedirect,
+    kHintPreloadSubresources,
     kHtmlWriterFilter,
     kIncludeJsSourceMaps,
     kInlineCss,
@@ -2989,9 +2990,7 @@ class RewriteOptions {
 
   // Returns true if this configuration turns on options that may need
   // the dependencies cohort to operate.
-  bool NeedsDependenciesCohort() const {
-    return Enabled(kExperimentHttp2);
-  }
+  bool NeedsDependenciesCohort() const;
 
  protected:
   // Helper class to represent an Option, whose value is held in some class T.

net/instaweb/rewriter/push_preload_filter_test.cc

@@ -42,7 +42,7 @@ class PushPreloadFilterTest : public RewriteTestBase {
  protected:
   void SetUp() override {
     RewriteTestBase::SetUp();
-    options()->EnableFilter(RewriteOptions::kExperimentHttp2);
+    options()->EnableFilter(RewriteOptions::kHintPreloadSubresources);
 
     // Setup pcache.
     pcache_ = rewrite_driver()->server_context()->page_property_cache();

net/instaweb/rewriter/rewrite_driver.cc

@@ -1329,7 +1329,7 @@ void RewriteDriver::AddPostRenderFilters() {
     // Remove excess whitespace in HTML.
     AddOwnedPostRenderFilter(new CollapseWhitespaceFilter(this));
   }
-  if (options()->NeedsDependenciesCohort()) {
+  if (options()->Enabled(RewriteOptions::kHintPreloadSubresources)) {
     AppendOwnedPreRenderFilter(new PushPreloadFilter(this));
   }
 

net/instaweb/rewriter/rewrite_filter_names.gperf

@@ -66,6 +66,7 @@ struct FilterMap {
 "fix_reflows",                       RewriteOptions::kFixReflows
 "flatten_css_imports",               RewriteOptions::kFlattenCssImports
 "flush_subresources",                RewriteOptions::kFlushSubresources
+"hint_preload_subresources",         RewriteOptions::kHintPreloadSubresources
 "in_place_optimize_for_browser",     RewriteOptions::kInPlaceOptimizeForBrowser
 "include_js_source_maps",            RewriteOptions::kIncludeJsSourceMaps
 "inline_css",                        RewriteOptions::kInlineCss

net/instaweb/rewriter/rewrite_options.cc

@@ -779,6 +779,8 @@ const RewriteOptions::FilterEnumToIdAndNameEntry
         {RewriteOptions::kFlushSubresources, "fs", "Flush Subresources"},
         {RewriteOptions::kHandleNoscriptRedirect, "hn",
          "Handles Noscript Redirects"},
+        {RewriteOptions::kHintPreloadSubresources, "hpsr",
+         "Hint Preload of Subresources"},
         {RewriteOptions::kHtmlWriterFilter, "hw", "Flushes html"},
         {RewriteOptions::kIncludeJsSourceMaps,
          RewriteOptions::kJavascriptMinSourceMapId, "Include JS Source Maps"},
@@ -5125,6 +5127,11 @@ HttpOptions RewriteOptions::ComputeHttpOptions() const {
   return options;
 }
 
+bool RewriteOptions::NeedsDependenciesCohort() const {
+  return Enabled(kExperimentHttp2) ||
+         Enabled(kHintPreloadSubresources);
+}
+
 bool RewriteOptions::CacheFragmentOption::SetFromString(
     StringPiece value, GoogleString* error_detail) {
   // The main thing here is that the fragment not contain '/' (the separator