[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..25aac17f2672c78b503f6bd9aa35b8be95a6bcc3 100644
--- a/base/message_loop/message_pump_mac.mm
+++ b/base/message_loop/message_pump_mac.mm
@@ -4,6 +4,7 @@
 
 #import "base/message_loop/message_pump_mac.h"
 
+#include <dlfcn.h>
 #import <Foundation/Foundation.h>
 
 #include <limits>
@@ -12,6 +13,7 @@
 #include "base/format_macros.h"
 #include "base/logging.h"
 #include "base/mac/scoped_cftyperef.h"
+#include "base/message_loop/timer_slack.h"
 #include "base/metrics/histogram.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
@@ -35,6 +37,33 @@ 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;
+
+  static dispatch_once_t get_timer_tolerance_function_ptr_once;
+  dispatch_once(&get_timer_tolerance_function_ptr_once, ^{
+      NSBundle* bundle =[NSBundle
+        bundleWithPath:@"/System/Library/Frameworks/CoreFoundation.framework"];
+      const char* path = [[bundle executablePath] fileSystemRepresentation];
+      CHECK(path);
+      void* library_handle = dlopen(path, RTLD_LAZY | RTLD_LOCAL);
+      CHECK(library_handle) << dlerror();
+      settimertolerance_function_ptr =
+          reinterpret_cast<CFRunLoopTimerSetTolerancePtr>(
+              dlsym(library_handle, "CFRunLoopTimerSetTolerance"));
+
+      dlclose(library_handle);
+  });
+
+  if (settimertolerance_function_ptr)
+    settimertolerance_function_ptr(timer, tolerance);
+}
+
 }  // namespace
 
 namespace base {
@@ -282,6 +311,7 @@ class MessagePumpInstrumentation {
 MessagePumpCFRunLoopBase::MessagePumpCFRunLoopBase()
     : delegate_(NULL),
       delayed_work_fire_time_(kCFTimeIntervalMax),
+      timer_slack_(base::TIMER_SLACK_NONE),
       nesting_level_(0),
       run_nesting_level_(0),
       deepest_nesting_level_(0),
@@ -438,6 +468,15 @@ 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 (timer_slack_ == TIMER_SLACK_MAXIMUM) {
+    SetTimerTolerance(delayed_work_timer_, delta.InSecondsF() * 0.5);
+  } else {
+    SetTimerTolerance(delayed_work_timer_, 0);
+  }
+}
+
+void MessagePumpCFRunLoopBase::SetTimerSlack(TimerSlack timer_slack) {
+  timer_slack_ = timer_slack;
 }
 
 // Called from the run loop.