Revert 172301 "Assert that we don't ever get a null layerTreeView."

This broke Android (dbg) webkit_unit_tests
WebViewTest.ChooseValueFromDateTimeChooser.

> Assert that we don't ever get a null layerTreeView.
> 
> We'd like to delete the null layerTreeView codepath as it's
> one of the few remaining that doesn't use accelerated
> compositing.
> 
> The crux of this patch is the ASSERT in WebViewImpl.cpp. The
> rest is just making the unittests pass by making sure they
> all use a WebViewClient that returns a non-null layerTreeView.
> 
> There's a few WebFrameTests that are crashing for some
> reason I don't understand. For now I've put in a
> BrokenNullLayerTreeViewWebViewClient to keep the tests
> passing. Will remove in a followup patch.
> 
> Also, WebFrameTest.pageScaleFactorScalesPaintClip and
> WebViewTest.SetBaseBackgroundColorAndBlendWithExistingContent
> break with this patch, but this is just exposing that these
> tests fail when they go down the compositing codepath, which
> all our shipping configurations do. Changing them to use the
> default WebViewClient makes them pass again.
> 
> Review URL: https://codereview.chromium.org/243783003

TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/247853004

git-svn-id: svn://svn.chromium.org/blink/trunk@172316 bbb929c8-8fbe-4397-9dbb-9b2b20218538