Fix for renderer crash in LinkHighlight::computeEnclosingCompositingLayer

It seems that the assumption in this function that the render layer will
have a ScrollableArea has changed. This leads to a crash attempting to
dereference it. It turns out an inline (no ScrollableArea) can be promoted
to a layer but also not draw its contents (e.g. visibility: hidden). 

Additionally, the old check that a layer that doesn't draw content must have
composited scrolling failed for promoted images so I removed the ASSERT added
a test for that too.

BUG=322591

Review URL: https://codereview.chromium.org/114173004

git-svn-id: svn://svn.chromium.org/blink/trunk@163870 bbb929c8-8fbe-4397-9dbb-9b2b20218538