amphtml: Migrate the use of user().error() to this.user().error()

What’s the issue

The plan is to eliminate user().error() call, and instead using: this.user().error(), or: this.ampdoc().user().error()

Initial problem

• I’m working on the user-error-reporting project, which reports user errors to publishers so that they can act upon.
• In order to exclude errors generated from iframe embed, we agreed to pass in a base-element whenever calling user() (see A4A errors in the doc): user(this).error().
• Currently, I’m having an {Element} passed in (see isFromEmbed). The element is now optional.
• In the end, we’d want to use {BaseElement} instead of {Element}, and make it a required param when using user().
• However, base-element.js and log.js will have circular dependency if I introduce base-element in my code in log.js: element.ownerDocument.defaultView != ampdoc.win. In addition, not every class/file has a base element to pass around.

Possible approach

• In base-element.js, introduce another user() field: user() { return user(this.element); } This would eliminate the circular dependency.
• Also introduce similar user() field in amp-doc, so that whenever base-element is not available, we can use amp-doc to check: this.ampdoc().user().error()

Discuss

• This will change everyone’s use of user() and possibly coding style (passing by ampdoc, etc.). Therefore we’re looking for everyone’s opinion on this design.