Fix for plugin printing regression.

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180220

[abarth-chromium, 2014-08-13 22:07:43]

LGTM

[Vitaly Buka (NO REVIEWS), 2014-08-13 22:35:52]

The CQ bit was checked by vitalybuka@chromium.org

[commit-bot: I haz the power, 2014-08-13 22:37:39]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/vitalybuka@chromium.org/476463002/20001

[commit-bot: I haz the power, 2014-08-14 01:46:16]

Committed patchset #2 (20001) as 180220

[adamk, 2014-08-14 16:05:25]

On 2014/08/14 at 01:46:16, commit-bot wrote:
> Committed patchset #2 (20001) as 180220

The test added in this patch crashes every time it's run on the bots. See:

http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Android%20%28Ne...

for example output and:

http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Android%20%28Ne...

for a broader picture.

But I'm not sure that reverting this change is safe because it says it's a
"fix". Any suggestions would be appreciated.

[adamk, 2014-08-14 16:06:29]

To be clear, the crashes occur only on the Android bot.

[abarth-chromium, 2014-08-14 16:13:27]

On 2014/08/14 at 16:06:29, adamk wrote:
> To be clear, the crashes occur only on the Android bot.

Please disable the test on Android.  I'll look in more detail.