Enable WebNavigationApiTest.CrossProcessHistory test.

This test was flaky due to a race condition in navigation code, which
has now been fixed. Let's try to enable it and see if the fix has helped
deflake this test.

Bug: 708139
Change-Id: I07c975eb6d6e207b6108f9f1cc6ed0e965981cde
Reviewed-on: https://chromium-review.googlesource.com/1102660Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567697}

chrome/browser/extensions/api/web_navigation/web_navigation_apitest.cc

@@ -507,8 +507,7 @@ IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, DISABLED_CrossProcessFragment) {
       << message_;
 }
 
-// crbug.com/708139.
-IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, DISABLED_CrossProcessHistory) {
+IN_PROC_BROWSER_TEST_F(WebNavigationApiTest, CrossProcessHistory) {
   ASSERT_TRUE(StartEmbeddedTestServer());
 
   // See crossProcessHistory/e.html.