Fix crash in ApplyStyleCommand::applyRelativeFontStyleChange()

Fix crash in ApplyStyleCommand::applyRelativeFontStyleChange(). The issue was
that we would traverse the DOM tree past the beyondEnd under some circumstances
and thus NodeTraversal::next() would return null unexpectedly. This CL adds a
check to make sure startNode != beyondEnd before traversing to avoid the
problem.

This CL also adds a few more assertions to catch similar issues more easily in
the future.

R=leviw@chromium.org, adamk
BUG=345950
TEST=editing/style/apply-style-crash2.html

Review URL: https://codereview.chromium.org/177093016

git-svn-id: svn://svn.chromium.org/blink/trunk@167845 bbb929c8-8fbe-4397-9dbb-9b2b20218538