Enable SitePerProcessHitTestDataGenerationBrowserTest

The test was previously disabled due to being flaky. Now that the fixing
patch was landed, we should enable this test again.

Bug: 869314
Change-Id: I7faf6103cd1d5232d058494fe3c4e3018fe96db6
Reviewed-on: https://chromium-review.googlesource.com/1173267Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Xianda Sun <sunxd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582695}

content/browser/site_per_process_hit_test_browsertest.cc

@@ -4744,7 +4744,7 @@ IN_PROC_BROWSER_TEST_P(SitePerProcessHitTestDataGenerationBrowserTest,
 }
 
 IN_PROC_BROWSER_TEST_P(SitePerProcessHitTestDataGenerationBrowserTest,
-                       DISABLED_ClippedOOPIFFastPath) {
+                       ClippedOOPIFFastPath) {
   if (!features::IsVizHitTestingEnabled())
     return;
   auto hit_test_data =