Fork of https://codereview.chromium.org/469833003/

Cannot print PDFs

In r180112, I removed some seemly dead code, but that code actually existed to
make some functions on PrintContext virtual. Instead of hacking in virtuals via
a subclass in the web layer, we can just make the functions virtual in the base
class.

I've also added OVERRIDE annotations to catch this sort of error at compile
time. These annotations required me to fix the return type of one of these
functions to match. Finally, I've added a test, which appears to be the first
unit test to exercise this codepath.

BUG=403467

Review URL: https://codereview.chromium.org/476463002

git-svn-id: svn://svn.chromium.org/blink/trunk@180220 bbb929c8-8fbe-4397-9dbb-9b2b20218538