[Match Spec] Form element pointer should not be updated w/in <template>

This CL updates Blink HTML Parser to better match the spec.
re: http://html5.org/tools/web-apps-tracker?from=8330&to=8331

The LayoutTest is based on WebKit change by rniwa@webkit.org:
https://trac.webkit.org/changeset/160182

Bonus:
- HTMLConstructionSite::setForm is merged into initFragmentParsing

BUG=326058

Review-Url: https://codereview.chromium.org/2143833003
Cr-Commit-Position: refs/heads/master@{#405065}
4 files changed