Reland RedirectNonUINonIOBrowserThreads to TaskScheduler by default on trunk

This reverts commit f07732ec
(https://codereview.chromium.org/2791933004)
and reapplies b6d0c9a0
(https://codereview.chromium.org/2690183002/).

BUG=653916, 707986
TBR=jam@chromium.org, rkaplow@chromium.org
Previously reviewed at https://codereview.chromium.org/2690183002/

Review-Url: https://codereview.chromium.org/2856583002
Cr-Commit-Position: refs/heads/master@{#468376}

base/threading/thread_restrictions.h

@@ -34,9 +34,11 @@ class Predictor;
 namespace content {
 class BrowserGpuChannelHostFactory;
 class BrowserGpuMemoryBufferManager;
+class BrowserMainLoop;
 class BrowserShutdownProfileDumper;
 class BrowserSurfaceViewManager;
 class BrowserTestBase;
+class CategorizedWorkerPool;
 class NestedMessagePumpAndroid;
 class ScopedAllowWaitForAndroidLayoutTests;
 class ScopedAllowWaitForDebugURL;
@@ -45,7 +47,6 @@ class SynchronousCompositor;
 class SynchronousCompositorBrowserFilter;
 class SynchronousCompositorHost;
 class TextInputClientMac;
-class CategorizedWorkerPool;
 }  // namespace content
 namespace dbus {
 class Bus;
@@ -185,6 +186,7 @@ class BASE_EXPORT ThreadRestrictions {
   friend class android_webview::AwFormDatabaseService;
   friend class android_webview::CookieManager;
   friend class base::StackSamplingProfiler;
+  friend class content::BrowserMainLoop;
   friend class content::BrowserShutdownProfileDumper;
   friend class content::BrowserSurfaceViewManager;
   friend class content::BrowserTestBase;

content/browser/browser_main_loop.cc

@@ -1259,19 +1259,20 @@ void BrowserMainLoop::ShutdownThreadsAndCleanUp() {
   service_manager_context_.reset();
   mojo_ipc_support_.reset();
 
+  {
+    base::ThreadRestrictions::ScopedAllowWait allow_wait_for_join;
+
     // Must be size_t so we can subtract from it.
     for (size_t thread_id = BrowserThread::ID_COUNT - 1;
-       thread_id >= (BrowserThread::UI + 1);
-       --thread_id) {
+         thread_id >= (BrowserThread::UI + 1); --thread_id) {
       // Find the thread object we want to stop. Looping over all valid
       // BrowserThread IDs and DCHECKing on a missing case in the switch
       // statement helps avoid a mismatch between this code and the
       // BrowserThread::ID enumeration.
       //
       // The destruction order is the reverse order of occurrence in the
-    // BrowserThread::ID list. The rationale for the order is as
-    // follows (need to be filled in a bit):
-    //
+      // BrowserThread::ID list. The rationale for the order is as follows (need
+      // to be filled in a bit):
       //
       // - The IO thread is the only user of the CACHE thread.
       //
@@ -1327,11 +1328,11 @@ void BrowserMainLoop::ShutdownThreadsAndCleanUp() {
     }
 
     // Close the blocking I/O pool after the other threads. Other threads such
-  // as the I/O thread may need to schedule work like closing files or flushing
-  // data during shutdown, so the blocking pool needs to be available. There
-  // may also be slow operations pending that will blcok shutdown, so closing
-  // it here (which will block until required operations are complete) gives
-  // more head start for those operations to finish.
+    // as the I/O thread may need to schedule work like closing files or
+    // flushing data during shutdown, so the blocking pool needs to be
+    // available. There may also be slow operations pending that will blcok
+    // shutdown, so closing it here (which will block until required operations
+    // are complete) gives more head start for those operations to finish.
     {
       TRACE_EVENT0("shutdown", "BrowserMainLoop::Subsystem:ThreadPool");
       BrowserThreadImpl::ShutdownThreadPool();
@@ -1340,6 +1341,7 @@ void BrowserMainLoop::ShutdownThreadsAndCleanUp() {
       TRACE_EVENT0("shutdown", "BrowserMainLoop::Subsystem:TaskScheduler");
       base::TaskScheduler::GetInstance()->Shutdown();
     }
+  }
 
   // Must happen after the IO thread is shutdown since this may be accessed from
   // it.

testing/variations/fieldtrial_testing_config.json

@@ -365,6 +365,7 @@
                 {
                     "name": "Default",
                     "params": {
+                        "RedirectNonUINonIOBrowserThreads": "true",
                         "RedirectSequencedWorkerPools": "true"
                     }
                 },