[Mac] Maximise timer slack for background tabs

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 891cc347448ff70b30f9357ff431d46aad613f59..d3b4151e95cc53e03ef3158994b8696a1b78d043 100644
--- a/base/message_loop/message_pump_mac.mm
+++ b/base/message_loop/message_pump_mac.mm
@@ -6,6 +6,7 @@
 
 #import <Foundation/Foundation.h>
 
+#include <dlfcn.h>
 #include <limits>
 #include <stack>
 
@@ -13,6 +14,7 @@
 #include "base/logging.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/metrics/histogram.h"
+#include "base/message_loop/timer_slack.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
 #include "base/time/time.h"
@@ -35,6 +37,31 @@ const CFTimeInterval kCFTimeIntervalMax =
 bool g_not_using_cr_app = false;
 #endif
 
+// Call through to CFRunLoopTimerSetTolerance(), which is only available on
+// OS X 10.9.
+void SetTimerTolerance(CFRunLoopTimerRef timer, CFTimeInterval tolerance) {
+  typedef void (*CFRunLoopTimerSetTolerancePtr)(CFRunLoopTimerRef timer,
+      CFTimeInterval tolerance);
+
+  static CFRunLoopTimerSetTolerancePtr settimertolerance_function_ptr = NULL;
+  static bool function_lookup_error = false;
+
+  if (function_lookup_error)
+    return;
+
+  if (!settimertolerance_function_ptr) {
+    settimertolerance_function_ptr =
+        reinterpret_cast<CFRunLoopTimerSetTolerancePtr>(
+            dlsym(RTLD_DEFAULT, "CFRunLoopTimerSetTolerance"));
+    if (!settimertolerance_function_ptr) {
+        function_lookup_error = true;
+        return;
+    }
+  }
+
+  settimertolerance_function_ptr(timer, tolerance);
+}
+
 }  // namespace
 
 namespace base {
@@ -438,6 +465,11 @@ void MessagePumpCFRunLoopBase::ScheduleDelayedWork(
   TimeDelta delta = delayed_work_time - TimeTicks::Now();
   delayed_work_fire_time_ = CFAbsoluteTimeGetCurrent() + delta.InSecondsF();
   CFRunLoopTimerSetNextFireDate(delayed_work_timer_, delayed_work_fire_time_);
+  if (GetTimerSlack() == TIMER_SLACK_MAXIMUM) {
+    SetTimerTolerance(delayed_work_timer_, delta.InSecondsF() * 0.5);
+  } else {
+    SetTimerTolerance(delayed_work_timer_, 0);
+  }
 }
 
 // Called from the run loop.