Cleanup feature to control the priority of the video capture service.

The Beta experiment shows that using USER_BLOCKING priority does not
negatively affect performance. In Slow Reports traces, we observed
that using BEST_EFFORT priority caused jank. Also, BEST_EFFORT is
not an appropriate priority to perform work that is visible to
the user. Therefore, this CL permanently changes the priority of
the video capture service to USER_BLOCKING. See
go/videocaptureservice-finch-june2020

Bug: 1066137
Change-Id: Iff96490fb90278159a7086fea7ec1523a9f78848
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276237
Auto-Submit: François Doray <fdoray@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785371}

services/video_capture/video_capture_service_impl.cc

@@ -33,17 +33,6 @@
 
 namespace video_capture {
 
-namespace {
-
-// Used to assess the impact of running the video capture service at background
-// priority. If the experiment confirms that running the video capture service
-// at background priority causes jank, change the code to use a foreground
-// priority by default. See https://crbug.com/1066137.
-const base::Feature kForegroundVideoCaptureService{
-    "ForegroundVideoCaptureService", base::FEATURE_DISABLED_BY_DEFAULT};
-
-}  // namespace
-
 // Intended usage of this class is to instantiate on any sequence, and then
 // operate and release the instance on the task runner exposed via
 // GetTaskRunner() via WeakPtrs provided via GetWeakPtr(). To this end,
@@ -52,13 +41,8 @@ const base::Feature kForegroundVideoCaptureService{
 class VideoCaptureServiceImpl::GpuDependenciesContext {
  public:
   GpuDependenciesContext() {
-    const base::TaskPriority priority =
-        base::FeatureList::IsEnabled(kForegroundVideoCaptureService)
-            ? base::TaskPriority::USER_BLOCKING
-            : base::TaskPriority::BEST_EFFORT;
-
     gpu_io_task_runner_ = base::ThreadPool::CreateSequencedTaskRunner(
-        {priority, base::MayBlock()});
+        {base::TaskPriority::USER_BLOCKING, base::MayBlock()});
   }
 
   ~GpuDependenciesContext() {

testing/variations/fieldtrial_testing_config.json

@@ -2789,28 +2789,6 @@
             ]
         }
     ],
-    "ForegroundVideoCaptureService": [
-        {
-            "platforms": [
-                "android",
-                "android_weblayer",
-                "android_webview",
-                "chromeos",
-                "ios",
-                "linux",
-                "mac",
-                "windows"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "ForegroundVideoCaptureService"
-                    ]
-                }
-            ]
-        }
-    ],
     "FormControlsRefresh": [
         {
             "platforms": [