Prerender: Re-enable PrerenderPageLoadMetricsObserverBrowserTest::

Activate_SpeculationRule

This test was disabled two years ago and we cannot access the test log
anymore.
To figure out if the issue has been fixed or not, this CL tentatively
re-enables the test.

To Gardener: feel free to revert this change and I'd appreciate it if
you could share some failure log!

Bug: 40842862
Change-Id: Ic30b305f804ab4a54eacd3a19a726af2103f6ea5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5574607
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Lingqi Chi <lingqi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1306686}

chrome/browser/page_load_metrics/observers/prerender_page_load_metrics_observer_browsertest.cc

@@ -201,14 +201,8 @@ class PrerenderPageLoadMetricsObserverBrowserTest
   base::test::ScopedFeatureList scoped_feature_list_;
 };
 
-// TODO(crbug.com/40842862): Re-enable this test
-#if BUILDFLAG(IS_MAC)
-#define MAYBE_Activate_SpeculationRule DISABLED_Activate_SpeculationRule
-#else
-#define MAYBE_Activate_SpeculationRule Activate_SpeculationRule
-#endif
 IN_PROC_BROWSER_TEST_F(PrerenderPageLoadMetricsObserverBrowserTest,
-                       MAYBE_Activate_SpeculationRule) {
+                       Activate_SpeculationRule) {
   ASSERT_TRUE(embedded_test_server()->Start());
 
   // Navigate to an initial page.