Retire rendering pipeline throttling runtime flag

Pipeline throttling for iframes with loading sheets shipped in M56
https://codereview.chromium.org/2488213005
https://www.chromestatus.com/feature/5640551701413888

The flag RenderingPipelineThrottlingLoadingIframes is no longer needed.

BUG=663193

Change-Id: Ieba672a8f7a8e3e0702a182aacf9931fec54a3d9
Reviewed-on: https://chromium-review.googlesource.com/1166747
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Ojan Vafai <ojan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582043}

third_party/blink/renderer/core/frame/local_frame_view.cc

@@ -286,9 +286,7 @@ void LocalFrameView::Reset() {
   // The compositor throttles the main frame using deferred commits, we can't
   // throttle it here or it seems the root compositor doesn't get setup
   // properly.
-  if (RuntimeEnabledFeatures::
-          RenderingPipelineThrottlingLoadingIframesEnabled())
-    lifecycle_updates_throttled_ = !GetFrame().IsMainFrame();
+  lifecycle_updates_throttled_ = !GetFrame().IsMainFrame();
   has_pending_layout_ = false;
   layout_scheduling_enabled_ = true;
   in_synchronous_post_layout_ = false;

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1020,10 +1020,6 @@
       name: "RenderingPipelineThrottling",
       status: "stable",
     },
-    {
-      name: "RenderingPipelineThrottlingLoadingIframes",
-      status: "stable",
-    },
     {
       name: "ReportingObserver",
       implied_by: ["DeprecationReporting", "InterventionReporting"],