Revert of Disabled flaky test AppBackgroundPageApiTest.NoJsBackgroundPage...

Revert of Disabled flaky test AppBackgroundPageApiTest.NoJsBackgroundPage (patchset #1 id:1 of https://codereview.chromium.org/2294113003/ )

Reason for revert:
The bug is now fixed so disabling this test is no longer needed.
See https://codereview.chromium.org/2293293002/

Original issue's description:
> Disabled flaky test AppBackgroundPageApiTest.NoJsBackgroundPage
>
> BUG=642482
> TBR=stanisc@chromium.org,nick@chromium.org
>
> Committed: https://crrev.com/940f8f135b20b2e5bca8c76ad036eb98e142def0
> Cr-Commit-Position: refs/heads/master@{#415620}

TBR=nick@chromium.org,battre@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=642482

Review-Url: https://codereview.chromium.org/2299803002
Cr-Commit-Position: refs/heads/master@{#415749}

chrome/browser/extensions/app_background_page_apitest.cc

@@ -304,8 +304,7 @@ IN_PROC_BROWSER_TEST_F(AppBackgroundPageApiTest, ManifestBackgroundPage) {
   UnloadExtension(extension->id());
 }
 
-// TODO(crbug.com/642482) Disabled test for flakyness.
-IN_PROC_BROWSER_TEST_F(AppBackgroundPageApiTest, DISABLED_NoJsBackgroundPage) {
+IN_PROC_BROWSER_TEST_F(AppBackgroundPageApiTest, NoJsBackgroundPage) {
   // Keep the task manager up through this test to verify that a crash doesn't
   // happen when window.open creates a background page that switches
   // RenderViewHosts. See http://crbug.com/165138.