Make WebGL backing array allocation use the buffer partition.

These are arrays of user-controlable scalars. As such, they
should be allocated from the buffer partition.

BUG=414883

Review URL: https://codereview.chromium.org/564213003

git-svn-id: svn://svn.chromium.org/blink/trunk@182514 bbb929c8-8fbe-4397-9dbb-9b2b20218538