mac: Add a Feature to control CanInvalidateCFRunLoopTimers()

This adds the MessagePumpMacTimerInvalidation feature to control whether
or not the MessagePumpMac's CFRunLoopTimer invalidation hack is enabled.
This will be used to A/B test its efficacy, now that MessagePumpMac has
switched to DoSomeWork().

Bug: 925998
Change-Id: I1e62d3a679c7590f4674a8d0f44b936b82856534
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036394Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750979}

base/message_loop/message_pump_mac.mm

@@ -10,6 +10,7 @@
 #include <memory>
 
 #include "base/auto_reset.h"
+#include "base/feature_list.h"
 #include "base/logging.h"
 #include "base/mac/call_with_eh_frame.h"
 #include "base/mac/scoped_cftyperef.h"
@@ -52,6 +53,9 @@ bool g_not_using_cr_app = false;
 // The MessagePump controlling [NSApp run].
 MessagePumpNSApplication* g_app_pump;
 
+Feature kMessagePumpTimerInvalidation{"MessagePumpMacTimerInvalidation",
+                                      FEATURE_ENABLED_BY_DEFAULT};
+
 // Various CoreFoundation definitions.
 typedef struct __CFRuntimeBase {
   uintptr_t _cfisa;
@@ -368,6 +372,10 @@ int MessagePumpCFRunLoopBase::GetModeMask() const {
 // uses public API to confirm that the private API changed the correct bit.
 // static
 bool MessagePumpCFRunLoopBase::CanInvalidateCFRunLoopTimers() {
+  if (!FeatureList::IsEnabled(kMessagePumpTimerInvalidation)) {
+    return false;
+  }
+
   CFRunLoopTimerContext timer_context = CFRunLoopTimerContext();
   timer_context.info = nullptr;
   ScopedCFTypeRef<CFRunLoopTimerRef> test_timer(