Remove the inherently flaky test
NoStatePrefetchBrowserTest.OpenTaskManagerBeforePrefetch and revert the
following revert:

> Reason for revert:
> "NoStatePrefetchBrowserTest.OpenTaskManagerBeforePrefetch" became flaky
> shortly after this landed.
>
> BUG=659697
> Original issue's description:
> > [NoStatePrefetch] Kill renderer after preload scanning
> >
> > The design doc is linked in the first bug. Main goal: remove Prerender.
> >
> > Currently NoState Prefetch uses the same logic to manage renderers as the
> > PrerenderManager. Prefetch code is still living in PrerenderManager, separating
> > the code from there is planned after we finalize on how much of the
> > functionality of PrerenderManager is needed.
> >
> > This is the first tweak in the renderer lifetime: the renderer asks to be killed
> > as soon as the main resource is fully preload-scanned and all possible
> > subresources are requested, special Prerendering FinalStatus is recorded to allow:
> > * testing
> > * provide a hint that a new prefetch can be started soon
> >
> > Browsertest changes:
> >
> > * TestPrerender now keeps FinalStatus for longer, to be able to verify that it
> >   is correct even after the TestPrerenderContents is destroyed.
> >
> > * Prefetch browsertests wait only for creation of PrerenderContents, Prerender
> >   tests have more waiting for page loads on top of that
> >
> > * PrerenderTestUrlImpl is removed to eliminate one hop through protected method
> >   in the parent class, which also simplified the prefetch side of the tests: no
> >   need to mention how many page loads to wait for
> >
> > * Disabled two tests that load a non-HTML document, correctly killing the
> >   renderer will be done in later changes.
> >
> > BUG=632361, 649632
> >
> > Committed: https://crrev.com/66351bd0a0fc8740749663fe765a301e1a175538
> > Cr-Commit-Position: refs/heads/master@{#427678}
> Committed: https://crrev.com/f69bf24f0861768f03628206668ddb84f55dc8d4
> Cr-Commit-Position: refs/heads/master@{#427974}

TBR=jochen@chromium.org,csharrison@chromium.org,mattcary@chromium.org,mkwst@chromium.org,pasko@chromium.org
BUG=632361, 649632

Review-Url: https://codereview.chromium.org/2455653005
Cr-Commit-Position: refs/heads/master@{#428027}