Created tracking spreadsheet.
https://docs.google.com/spreadsheets/d/1s1X0a2IfhzaNi6TsjCMBEOiF1GyxmMhAuluCIodUMHw/edit#gid=0

WebUI is using Polymer V1, but has a plan to migrate to Polymer V2.

What’s the deadline for this? We’d like to plan migrations of our legacy apps

FYI, the spreadsheet linked from c#1 is open for editing by everyone with
@chromium.org account.  If you need access, feel free to request it.

Re c#7, our tentative plan is disabling the API at April 2019.

This bug is for preparing "intent to deprecate and remove Shadow DOM V0".
The API usage is still high, and there's uncertainty about how we can
drive the usage on the web down, but at least we would like to have a plan
for Chrome's internal users to migrate to something else before then.

Transferring the ownership to yoichio@.