Add devtools trace events for DOM load notifications

BUG=361045

Review URL: https://codereview.chromium.org/252173002

git-svn-id: svn://svn.chromium.org/blink/trunk@172904 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Oilpan: Determine if a class is a GarbageCollectedMixin via its pure-virtual interface.

A class can inherit from multiple GarbageCollectedMixin bases which causes a cast from that type to GarbageCollectedMixin to be ambiguous. Our use of multiple bases is however not ambiguous because GarbageCollectedMixin only defines a pure-virtual interface. Instead of using virtual inheritance, we avoid the cast altogether and determine mixins by testing against its pure-virtual interface.

R=ager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/255063008

git-svn-id: svn://svn.chromium.org/blink/trunk@172905 bbb929c8-8fbe-4397-9dbb-9b2b20218538