Avoid WTF::Vector::at() and operator[] in core/dom.

at() and operator[] are slow due to RELEASE_ASSERT. We can avoid the slowness by
range-based |for|.

This CL has no behavior changes except runtime performance.

BUG=668300

Review-Url: https://codereview.chromium.org/2553343002
Cr-Commit-Position: refs/heads/master@{#436875}
17 files changed