[Mac] Execute as many MessageLoop tasks as possible.

base/message_loop/message_pump_mac.mm

Index: base/message_loop/message_pump_mac.mm
diff --git a/base/message_loop/message_pump_mac.mm b/base/message_loop/message_pump_mac.mm
index 0ab9ab7c467de5128be33444db018ce73d369f7d..205bd3e2f229c6c424fe55bc584fb21a3bfbfc06 100644
--- a/base/message_loop/message_pump_mac.mm
+++ b/base/message_loop/message_pump_mac.mm
@@ -553,10 +553,40 @@ bool MessagePumpCFRunLoopBase::RunWork() {
   // released promptly even in the absence of UI events.
   MessagePumpScopedAutoreleasePool autorelease_pool(this);
 
-  // Call DoWork and DoDelayedWork once, and if something was done, arrange to
-  // come back here again as long as the loop is still running.
-  bool did_work = delegate_->DoWork();
-  bool resignal_work_source = did_work;
+  // Allocate a time slice in which as many tasks as possible will be run.
+  // Performing only one task per run loop tick is potentially inefficient in
+  // the face of a large backlog of pending tasks. https://crbug.com/264886
+  base::TimeTicks now = base::TimeTicks::Now();
+  base::TimeTicks time_slice_end = now + base::TimeDelta::FromMilliseconds(50);
+  int ran_task_count = 0;
+  bool did_work = false, did_work_last_slice = true;
+  for ( ;
+       now < time_slice_end && did_work_last_slice;
+       now = base::TimeTicks::Now()) {
+    quit_was_called_ = false;
+    did_work_last_slice = delegate_->DoWork();
+    did_work |= did_work_last_slice;
+    if (quit_was_called_)
+      break;
+    if (did_work_last_slice)
+      ++ran_task_count;
+  }
+
+  // Record how many tasks were accomplished in the time slice, and by how
+  // much the work overran the time slice deadline.
+  UMA_HISTOGRAM_CUSTOM_TIMES("MessagePumpMac.TimeSliceOverRun",
+                             time_slice_end - base::TimeTicks::Now(),
+                             base::TimeDelta::FromMilliseconds(1),
+                             base::TimeDelta::FromSeconds(5),
+                             100);
+  UMA_HISTOGRAM_COUNTS("MessagePumpMac.TimeSliceTaskCount", ran_task_count);
+
+  if (quit_was_called_)
+    return false;
+
+  // If the time slice expired and work was done in the last fraction of the
+  // time slice, arrange to come back here to execute more tasks.
+  bool resignal_work_source = did_work_last_slice;
 
   TimeTicks next_time;
   delegate_->DoDelayedWork(&next_time);
@@ -795,6 +825,7 @@ void MessagePumpCFRunLoop::DoRun(Delegate* delegate) {
 
 // Must be called on the run loop thread.
 void MessagePumpCFRunLoop::Quit() {
+  quit_was_called_ = true;
   // Stop the innermost run loop managed by this MessagePumpCFRunLoop object.
   if (nesting_level() == run_nesting_level()) {
     // This object is running the innermost loop, just stop it.
@@ -849,6 +880,7 @@ void MessagePumpNSRunLoop::DoRun(Delegate* delegate) {
 }
 
 void MessagePumpNSRunLoop::Quit() {
+  quit_was_called_ = true;
   keep_running_ = false;
   CFRunLoopSourceSignal(quit_source_);
   CFRunLoopWakeUp(run_loop());
@@ -923,6 +955,7 @@ void MessagePumpNSApplication::DoRun(Delegate* delegate) {
 }
 
 void MessagePumpNSApplication::Quit() {
+  quit_was_called_ = true;
   if (!running_own_loop_) {
     [[NSApplication sharedApplication] stop:nil];
   } else {