[bfcache] Add ability to enable SW support from chrome://flags

Add chrome://flags entry to allow pages controlled by a Service
Worker to be put into back-forward.

This support is highly experimental, so please use with care.

In order to make this possible, BackForwardCacheWithServiceWorker
feature is replaced with a parameter on BackForwardCache parameter.

R=arthursonzogni@chromium.org

Change-Id: I741aefed9a54f87a4c6ece2003be0e8664ddd470
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807328
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697335}

chrome/browser/about_flags.cc

@@ -1332,6 +1332,17 @@ const FeatureEntry::FeatureVariation kQuietNotificationPromptsVariations[] = {
 };
 #endif  // !OS_ANDROID
 
+// TODO(crbug.com/991082): Remove after proper service worker support for
+// back-forward cache is implemented.
+const FeatureEntry::FeatureParam kBackForwardCache_ServiceWorkerSupport[] = {
+    {"service_worker_supported", "true"},
+};
+
+const FeatureEntry::FeatureVariation kBackForwardCacheVariations[] = {
+    {" even for ServiceWorker-controlled pages",
+     kBackForwardCache_ServiceWorkerSupport, 1, nullptr},
+};
+
 // RECORDING USER METRICS FOR FLAGS:
 // -----------------------------------------------------------------------------
 // The first line of the entry is the internal name.
@@ -4537,7 +4548,9 @@ const FeatureEntry kFeatureEntries[] = {
 
     {"back-forward-cache", flag_descriptions::kBackForwardCacheName,
      flag_descriptions::kBackForwardCacheDescription, kOsAll,
-     FEATURE_VALUE_TYPE(features::kBackForwardCache)},
+     FEATURE_WITH_PARAMS_VALUE_TYPE(features::kBackForwardCache,
+                                    kBackForwardCacheVariations,
+                                    "BackForwardCache")},
 
     // NOTE: Adding a new flag requires adding a corresponding entry to enum
     // "LoginCustomFlags" in tools/metrics/histograms/enums.xml. See "Flag

content/browser/back_forward_cache_browsertest.cc

@@ -51,14 +51,13 @@ class BackForwardCacheBrowserTest : public ContentBrowserTest {
         switches::kUseFakeUIForMediaStream);
     base::CommandLine::ForCurrentProcess()->AppendSwitch(
         switches::kIgnoreCertificateErrors);
-    feature_list_.InitWithFeatures(FeaturesToEnable(), {});
+    feature_list_.InitAndEnableFeatureWithParameters(
+        features::kBackForwardCache, GetFeatureParams());
 
     ContentBrowserTest::SetUpCommandLine(command_line);
   }
 
-  virtual std::vector<base::Feature> FeaturesToEnable() {
-    return std::vector<base::Feature>({features::kBackForwardCache});
-  }
+  virtual base::FieldTrialParams GetFeatureParams() { return {}; }
 
   void SetUpOnMainThread() override {
     host_resolver()->AddRule("*", "127.0.0.1");
@@ -1828,11 +1827,8 @@ class BackForwardCacheBrowserTestWithServiceWorkerEnabled
   ~BackForwardCacheBrowserTestWithServiceWorkerEnabled() override {}
 
  protected:
-  std::vector<base::Feature> FeaturesToEnable() override {
-    std::vector<base::Feature> result =
-        BackForwardCacheBrowserTest::FeaturesToEnable();
-    result.push_back(kBackForwardCacheWithServiceWorker);
-    return result;
+  base::FieldTrialParams GetFeatureParams() override {
+    return {{"service_worker_supported", "true"}};
   }
 };
 

content/browser/frame_host/back_forward_cache.cc

@@ -11,6 +11,7 @@
 #include "content/browser/frame_host/render_frame_host_impl.h"
 #include "content/browser/renderer_host/render_view_host_impl.h"
 #include "content/common/page_messages.h"
+#include "content/public/common/content_features.h"
 #include "content/public/common/navigation_policy.h"
 #include "net/http/http_status_code.h"
 #include "third_party/blink/public/common/scheduler/web_scheduler_tracked_feature.h"
@@ -91,6 +92,12 @@ bool CanStoreRenderFrameHost(RenderFrameHostImpl* rfh,
   return true;
 }
 
+bool IsServiceWorkerSupported() {
+  static constexpr base::FeatureParam<bool> service_worker_supported(
+      &features::kBackForwardCache, "service_worker_supported", false);
+  return service_worker_supported.Get();
+}
+
 uint64_t GetDisallowedFeatures() {
   // TODO(lowell): Finalize disallowed feature list, and test for each
   // disallowed feature.
@@ -120,7 +127,7 @@ uint64_t GetDisallowedFeatures() {
 
   uint64_t result = kAlwaysDisallowedFeatures;
 
-  if (!base::FeatureList::IsEnabled(kBackForwardCacheWithServiceWorker)) {
+  if (!IsServiceWorkerSupported()) {
     result |=
         ToFeatureBit(WebSchedulerTrackedFeature::kServiceWorkerControlledPage);
   }
@@ -129,9 +136,6 @@ uint64_t GetDisallowedFeatures() {
 
 }  // namespace
 
-const base::Feature kBackForwardCacheWithServiceWorker = {
-    "BackForwardCacheWithServiceWorker", base::FEATURE_DISABLED_BY_DEFAULT};
-
 BackForwardCache::BackForwardCache() : weak_factory_(this) {}
 BackForwardCache::~BackForwardCache() = default;
 

content/browser/frame_host/back_forward_cache.h

@@ -147,12 +147,6 @@ class CONTENT_EXPORT BackForwardCache {
   DISALLOW_COPY_AND_ASSIGN(BackForwardCache);
 };
 
-// TODO(crbug.com/991082): We need to implement frozen frame enumeration
-// before we can properly support pages with ServiceWorker in back-forward
-// cache. This flag allows to bypass this restriction for local testing.
-// Remove after ServiceWorker support is implemented.
-CONTENT_EXPORT extern const base::Feature kBackForwardCacheWithServiceWorker;
-
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_FRAME_HOST_BACK_FORWARD_CACHE_H_