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.

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/256463002

git-svn-id: svn://svn.chromium.org/blink/trunk@172552 bbb929c8-8fbe-4397-9dbb-9b2b20218538