This makes it no longer necessary to ensure that V8 and Blink have non-colliding
tags, which makes it easier for them to evolve independently, and also makes
the wire format more suitable for other V8 embedders, who would not
necessarily be surveyed before V8 introduced a new tag that might collide
with theirs.

BUG=chromium:686159

Review-Url: https://codereview.chromium.org/2709023003
Cr-Commit-Position: refs/heads/master@{#43466}