The syntax class whitelist is a feature that restricts the DOM elements
created for syntax highlighting only to classes which have CSS styles.
However, the test for this whitelist was being applied at the wrong
place, and would ignore whitelisted classes that were nested inside
non-whitelisted classes.
With this change, the location of the whitelist test is fixed, and a
unit test is added to encode the correct behavior.
Bug: Issue 4578
Change-Id: Ic3631b0fb44dc2c691069b8134ad88a55feb977a