Prerender: Enable PrerenderBrowserTest.PrerenderBlankIframe

To sheriffs: Feel free to revert this CL if the test is flaky.

The CL author tried to reproduce this on local environments but
failed. For further investigation, this CL enables the test again to see
how it works on the try bots.

Bug: 1185965
Change-Id: Ie7f65ef094a815e72b93091c8b3d0ab9b361e593
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2850329
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#876111}

content/browser/prerender/prerender_browsertest.cc

@@ -802,8 +802,7 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderIframe) {
 
 // Blank <iframe> is a special case. Tests that the blank iframe knows the
 // prerendering state as well.
-// TODO(https://crbug.com/1185965): This test is disabled for flakiness.
-IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, DISABLED_PrerenderBlankIframe) {
+IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderBlankIframe) {
   TestHostPrerenderingState(GetUrl("/page_with_blank_iframe.html"));
 }