Update rendering classes to use OVERRIDE / FINAL when needed

Update rendering classes to use OVERRIDE / FINAL when needed. Those are useful for
error checking or performance when the compiler supports it.

R=esprehn, abarth
BUG=333316

Review URL: https://codereview.chromium.org/129173004

git-svn-id: svn://svn.chromium.org/blink/trunk@164917 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Put the BeforeLoad event behind a runtime flag

This CL puts the BeforeLoad event behind a runtime flag with the eventual goal
of removing the feature entirely (see blink-dev discussion for more details).
The flag is enabled during testing so that we can continue to test and fuzz
beforeload while this CL works its way through the release pipeline. If this CL
doesn't cause compatibility problems, we can actually remove the code.

R=esprehn@chromium.org
BUG=333318

Review URL: https://codereview.chromium.org/130573004

git-svn-id: svn://svn.chromium.org/blink/trunk@164918 bbb929c8-8fbe-4397-9dbb-9b2b20218538