Add assert against nesting DrawingRecorders.

We should never instantiate a DrawingRecorder while another
is active. Previously, we were doing this in quite a few places,
which masked both missing drawing recorders and created nested
SKPictures.

Among the places we weren't properly creating DrawingRecorders
were InlineBoxes. Ultimately, we'll need to allow DrawingRecorders
to point to either an InlineBox *or* a Renderer, as otherwise we'll
potentially get invalidations wrong for inlines, as InlineBoxes
have a many to one relationship with Renderers.

Review URL: https://codereview.chromium.org/680273008

git-svn-id: svn://svn.chromium.org/blink/trunk@184690 bbb929c8-8fbe-4397-9dbb-9b2b20218538