[blink scheduler] Allow aggressive throttling on pages using a WebSocket.

Bug: 1121725
Change-Id: I6ae687a4ed0324d32b76b4ec4d60b7b948245d91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376445Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810696}

third_party/blink/renderer/modules/websockets/websocket_channel_impl.cc

@@ -35,6 +35,7 @@
 
 #include "base/callback.h"
 #include "base/compiler_specific.h"
+#include "base/feature_list.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
 #include "base/util/type_safety/strong_alias.h"
@@ -81,6 +82,17 @@ enum WebSocketOpCode {
   kOpCodeBinary = 0x2,
 };
 
+// When enabled, a page can be aggressively throttled even if it uses a
+// WebSocket. Aggressive throttling does not affect the execution of WebSocket
+// event handlers, so there is little reason to disable it on pages using a
+// WebSocket.
+//
+// TODO(crbug.com/1121725): Cleanup this feature once field experiments confirm
+// that the opt-out can be removed.
+const base::Feature kAllowAggressiveThrottlingWithWebSocket{
+    "AllowAggressiveThrottlingWithWebSocket",
+    base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace
 
 class WebSocketChannelImpl::BlobLoader final
@@ -205,8 +217,12 @@ bool WebSocketChannelImpl::Connect(const KURL& url, const String& protocol) {
   if (auto* scheduler = execution_context_->GetScheduler()) {
     feature_handle_for_scheduler_ = scheduler->RegisterFeature(
         SchedulingPolicy::Feature::kWebSocket,
-        {SchedulingPolicy::DisableAggressiveThrottling(),
-         SchedulingPolicy::RecordMetricsForBackForwardCache()});
+        base::FeatureList::IsEnabled(kAllowAggressiveThrottlingWithWebSocket)
+            ? SchedulingPolicy{SchedulingPolicy::
+                                   RecordMetricsForBackForwardCache()}
+            : SchedulingPolicy{
+                  SchedulingPolicy::DisableAggressiveThrottling(),
+                  SchedulingPolicy::RecordMetricsForBackForwardCache()});
   }
 
   if (MixedContentChecker::IsMixedContent(