Revert "Disable flaky BasicSelectionIsolatedScrollMainframe"

This reverts commit 3d6ada5e.

Reason for revert: wjmaclean@ landed potential fix in https://crrev.com/532661.
Let's see if it worked.

Original change's description:
> Disable flaky BasicSelectionIsolatedScrollMainframe
> 
> TouchSelectionForCrossProcessFramesTests/TouchSelectionControllerClientAuraSiteIsolationTest.BasicSelectionIsolatedScrollMainframe/1 fails on all the platforms.
> 
> TBR=tdresser@chromium.org
> 
> Bug: 805017
> Change-Id: I1ea7c82a4c8fc2fa8e83c83ac47ef65f98cd985f
> Reviewed-on: https://chromium-review.googlesource.com/888753
> Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
> Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#532122}

TBR=vasilii@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 805017
Change-Id: I59c74f106b2c04632e0826512e75c938587eccbe
Reviewed-on: https://chromium-review.googlesource.com/892203Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532708}

content/browser/renderer_host/input/touch_selection_controller_client_aura_browsertest.cc

@@ -477,9 +477,8 @@ IN_PROC_BROWSER_TEST_P(TouchSelectionControllerClientAuraSiteIsolationTest,
   EXPECT_FALSE(ui::TouchSelectionMenuRunner::GetInstance()->IsRunning());
 }
 
-// https://crbug.com/805017
 IN_PROC_BROWSER_TEST_P(TouchSelectionControllerClientAuraSiteIsolationTest,
-                       DISABLED_BasicSelectionIsolatedScrollMainframe) {
+                       BasicSelectionIsolatedScrollMainframe) {
   GURL test_url(embedded_test_server()->GetURL(
       "a.com", "/cross_site_iframe_factory.html?a(a)"));
   EXPECT_TRUE(NavigateToURL(shell(), test_url));