Rename v0 custom element flags to V0

This patch renames flags for v0 custom element to have "V0" in their
names, and the same changes to getter/setter of such flags, in order
to distinguish them from flags for custom element v1.

BUG=594918

Review-Url: https://codereview.chromium.org/1982003002
Cr-Commit-Position: refs/heads/master@{#394004}
9 files changed