Remove experiment for overriding UI thread priority

Remove the experiment for overriding UI thread priority, making the new
behavior the default.

The data from UMA[1] shows a (statistically insignificant) 8%
improvement in Event.Latency.ScrollBegin.Touch.TimeToScrollUpdateSwapBegin2
(99%ile) and and an 8% drop in
Startup.Android.Cold.TimeToFirstContentfulPaint.Tabbed (also 99%ile).

BUG=841804

[1] https://uma.googleplex.com/p/chrome/variations/?sid=b286e568c610608d39f5d557480c45c5

Change-Id: I55fe266c0101cf4b120c098e90d08bfde7e158a1
Reviewed-on: https://chromium-review.googlesource.com/1165345Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581517}

content/browser/browser_main_loop.cc

@@ -642,8 +642,7 @@ int BrowserMainLoop::EarlyInitialization() {
   // Up the priority of the UI thread unless it was already high (since recent
   // versions of Android (O+) do this automatically).
   if (base::PlatformThread::GetCurrentThreadPriority() <
-          base::ThreadPriority::DISPLAY ||
-      base::FeatureList::IsEnabled(features::kOverrideUIThreadPriority)) {
+      base::ThreadPriority::DISPLAY) {
     base::PlatformThread::SetCurrentThreadPriority(
         base::ThreadPriority::DISPLAY);
   }

content/public/common/content_features.cc

@@ -666,12 +666,6 @@ const base::Feature kMacV2Sandbox{"MacV2Sandbox",
                                   base::FEATURE_ENABLED_BY_DEFAULT};
 #endif  // defined(OS_MACOSX)
 
-#if defined(OS_ANDROID) || defined(OS_CHROMEOS)
-// Always set the UI thread's priority to DISPLAY.
-const base::Feature kOverrideUIThreadPriority{
-    "OverrideUIThreadPriority", base::FEATURE_DISABLED_BY_DEFAULT};
-#endif  // defined(OS_ANDROID) || defined(OS_CHROMEOS)
-
 enum class VideoCaptureServiceConfiguration {
   kEnabledForOutOfProcess,
   kEnabledForBrowserProcess,

content/public/common/content_features.h

@@ -156,10 +156,6 @@ CONTENT_EXPORT extern const char kWebXrRenderPathParamValueSharedBuffer[];
 CONTENT_EXPORT extern const base::Feature kWebUIPolymer2;
 #endif  // !defined(OS_ANDROID)
 
-#if defined(OS_ANDROID) || defined(OS_CHROMEOS)
-CONTENT_EXPORT extern const base::Feature kOverrideUIThreadPriority;
-#endif  // defined(OS_ANDROID) || defined(OS_CHROMEOS)
-
 #if defined(OS_MACOSX)
 CONTENT_EXPORT extern const base::Feature kDeviceMonitorMac;
 CONTENT_EXPORT extern const base::Feature kIOSurfaceCapturer;