Allocate stack item for fragment context elements once

Instead of allocating stack items for fragment context elements on
demand, do it once when creating the fragment context and re-use the
stack item during parsing. This avoids excessive calls to malloc when
parsing for innerHTML.

BUG=318711

Review URL: https://codereview.chromium.org/68893014

git-svn-id: svn://svn.chromium.org/blink/trunk@162047 bbb929c8-8fbe-4397-9dbb-9b2b20218538