Remove a level of indirection in HTMLCollection.

It's not necessary to give HeapVector a separate allocation (itself, it's
only a pointer and two integers). Fixing this requires us to make the code
const-correct. While here, it's easy to replace repeated push_back with
appendVector, which is more efficient.

Review-Url: https://codereview.chromium.org/2765653002
Cr-Commit-Position: refs/heads/master@{#458496}
3 files changed