Adjust logic for UI thread priority

This patch adjusts the logic for UI thread priority:

1) The priority is only increased to DISPLAY unless it was lower to
   begin with. Recent versions of Android (O+) automatically set the top
   activity's UI thread's niceness to TOP_APP_PRIORITY_BOOST (-10), so
   by setting the priority to DISPLAY (-4) we were in fact lowering the
   priority.

2) The priority is increased earlier in the start-up flow to minimize
   the chances of descheduling.

We also add a field trial for the first change to evaluate its impact.

BUG=841804
TEST=Manual inspection

Change-Id: I3ec70042be86f2b88bb1ae3a2dd8cba7d87eb807
Reviewed-on: https://chromium-review.googlesource.com/1136648
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576079}

content/browser/browser_main_loop.cc

@@ -638,6 +638,17 @@ int BrowserMainLoop::EarlyInitialization() {
         command_line->GetSwitchValueASCII(switches::kDisableFeatures));
   }
 
+#if defined(OS_ANDROID) || defined(OS_CHROMEOS)
+  // 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::PlatformThread::SetCurrentThreadPriority(
+        base::ThreadPriority::DISPLAY);
+  }
+#endif  // defined(OS_ANDROID) || defined(OS_CHROMEOS)
+
 #if defined(OS_MACOSX) || defined(OS_LINUX) || defined(OS_CHROMEOS) || \
     defined(OS_ANDROID)
   // We use quite a few file descriptors for our IPC as well as disk the disk
@@ -1238,10 +1249,6 @@ int BrowserMainLoop::BrowserThreadsStarted() {
 #endif
 
   HistogramSynchronizer::GetInstance();
-#if defined(OS_ANDROID) || defined(OS_CHROMEOS)
-  // Up the priority of the UI thread.
-  base::PlatformThread::SetCurrentThreadPriority(base::ThreadPriority::DISPLAY);
-#endif
 
   // cc assumes a single client name for metrics in a process, which is
   // is inconsistent with single process mode where both the renderer and

content/public/common/content_features.cc

@@ -649,6 +649,12 @@ const base::Feature kMacV2Sandbox{"MacV2Sandbox",
                                   base::FEATURE_DISABLED_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)
+
 bool IsVideoCaptureServiceEnabledForOutOfProcess() {
 #if defined(OS_ANDROID)
   return false;

content/public/common/content_features.h

@@ -153,6 +153,10 @@ 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;