Several improvements to error reporting (ampproject#4878)

- thread through canary state into 3p iframe and report it.
- change throttling of error reporting by another factor of 10 for general traffic.

Unrelated: Forbid usage of `Object.assign` in 3p code as this showed up in error reports.

Fix js error that occurs when a extraUrlParams var has array value. (a…

…mpproject#4797)

This regression was introduced in ampproject#4576.

Fix js error that occurs when a extraUrlParams var has array value. (a…

…mpproject#4797)

This regression was introduced in ampproject#4576.

Started passing vendor requests through complete analytics lifecycle. (…

…ampproject#4668)

* Started passing vendor requests through complete analytics lifecycle.

As a result:
- extraUrlParams will get inserted into the urls if present.
- variables that are defined in the config will be inserted into the
  requests.

Fixes ampproject#3837

Ramp A4A experiment back to 5% of canary. (ampproject#4608)

Too much false positives (ampproject#4489)

Too much false positives (ampproject#4489)

Re-Fix build being called before element is ready to build. (ampproje…

…ct#4476)

Revert "Fix build being called before element is ready to build." (am…

…pproject#4467)

AMP slidescroll experiment: Hide Scrollbars (ampproject#4403)