Introduce OffMainThreadFetch feature.

This is the first patch for the off-main-thread fetch project.
https://goo.gl/hgmQbP

We will implement this under "--enable-features=OffMainThreadFetch" flag.

I will submit https://codereview.chromium.org/2799883002, when it will be ready to test in Chrome.

BUG=443374

Review-Url: https://codereview.chromium.org/2798083004
Cr-Commit-Position: refs/heads/master@{#462822}

content/public/common/content_features.cc

@@ -140,6 +140,10 @@ const base::Feature kNotificationContentImage{"NotificationContentImage",
 const base::Feature kMainThreadBusyScrollIntervention{
     "MainThreadBusyScrollIntervention", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Experimental resource fetch optimizations for workers. See crbug.com/443374
+const base::Feature kOffMainThreadFetch{"OffMainThreadFetch",
+                                        base::FEATURE_DISABLED_BY_DEFAULT};
+
 // Origin Trials for controlling access to feature/API experiments.
 const base::Feature kOriginTrials{"OriginTrials",
                                   base::FEATURE_ENABLED_BY_DEFAULT};

content/public/common/content_features.h

@@ -43,6 +43,7 @@ CONTENT_EXPORT extern const base::Feature kMediaStreamOldVideoConstraints;
 CONTENT_EXPORT extern const base::Feature kMemoryCoordinator;
 CONTENT_EXPORT extern const base::Feature kNotificationContentImage;
 CONTENT_EXPORT extern const base::Feature kMainThreadBusyScrollIntervention;
+CONTENT_EXPORT extern const base::Feature kOffMainThreadFetch;
 CONTENT_EXPORT extern const base::Feature kOriginTrials;
 CONTENT_EXPORT extern const base::Feature kParallelDownloading;
 CONTENT_EXPORT extern const base::Feature kPassiveDocumentEventListeners;