Land layer squashing behind a flag

This patch adds appropriate ASSERTs and flag
checks to ensure that the feature does not affect
tip of tree when the flag is disabled.

Tests and bugs will be added in upcoming patches.

BUG=261605

Review URL: https://codereview.chromium.org/88863002

git-svn-id: svn://svn.chromium.org/blink/trunk@163777 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Enforce HTML range restriction on setting unsigned attribute values.

The HTML spec requires that reflected attributes over unsigned longs
are kept to a limited range [0, 2^31) on setting & getting,

   http://www.whatwg.org/specs/web-apps/current-work/#reflect

No such clamping was in place for the upper bound; enforce it when
setting the unsigned long attribute.

R=
BUG=316122

Review URL: https://codereview.chromium.org/99443005

git-svn-id: svn://svn.chromium.org/blink/trunk@163778 bbb929c8-8fbe-4397-9dbb-9b2b20218538