Oilpan: remove finalization-order issue with WebGLVertexArrayObjectOES.

The code is clearing a heap allocated vector in a destructor. That can
lead to a memset of the backing. However, the backing could have been
already finalized. This is actually safe since it is just setting
memory on a freelist to zero (which it is already in release build)
but we should avoid touching already finalized objects. ASAN builds
catch this.

R=oilpan-reviews@chromium.org, sigbjornf@opera.com

Review URL: https://codereview.chromium.org/489363002

git-svn-id: svn://svn.chromium.org/blink/trunk@180730 bbb929c8-8fbe-4397-9dbb-9b2b20218538