Oilpan: Fix isAlive dispatching for GarbageCollectedMixins.

Update the test so that adjustment is needed and use a weak hash set
of mixin pointers.

I realized this didn't work when I needed to use it for the
Node hierarchy.

R=erik.corry@gmail.com, kouhei@chromium.org, oilpan-reviews@chromium.org, vegorov@chromium.org

Review URL: https://codereview.chromium.org/222653003

git-svn-id: svn://svn.chromium.org/blink/trunk@170688 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add a missing null check to LinkStyle::process()

This is a speculative fix for crashes that are reported from wild.

BUG=358844
R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/220723012

git-svn-id: svn://svn.chromium.org/blink/trunk@170689 bbb929c8-8fbe-4397-9dbb-9b2b20218538