These are properties that may used internally by LibWeb when resolving
style values, but may not be set by external stylesheets. For example,
'background-repeat' may be a two-value CSS property that internally
translates to 'background-repeat-x' and 'background-repeat-y'.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
