[Extensions] Re-enable a ScrollbarTest.

This test was disabled because it was flaky on Mac.

Testing indicates it's not flaky on any other platform
and is only flaky on the mac_chromium_10.12_rel_ng bot.

Bug: 988934
Change-Id: Iaa028f7792906db6ef7554f4a824f1810f706a1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393007Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807917}

chrome/browser/ui/views/extensions/extension_install_dialog_view_browsertest.cc

@@ -164,7 +164,13 @@ IN_PROC_BROWSER_TEST_F(ScrollbarTest, LongPromptScrollbar) {
 
 // Tests that a scrollbar isn't shown for this regression case.
 // See crbug.com/385570 for details.
-IN_PROC_BROWSER_TEST_F(ScrollbarTest, DISABLED_ScrollbarRegression) {
+// TODO(http://crbug.com/988934): Flaky on some Mac release bots.
+#if defined(OS_MAC) && defined(NDEBUG)
+#define MAYBE_ScrollbarRegression DISABLED_ScrollbarRegression
+#else
+#define MAYBE_ScrollbarRegression ScrollbarRegression
+#endif
+IN_PROC_BROWSER_TEST_F(ScrollbarTest, MAYBE_ScrollbarRegression) {
   base::string16 permission_string(base::ASCIIToUTF16(
       "Read and modify your data on *.facebook.com"));
   PermissionMessages permissions;