Predefined escapers (i.e. "html" and "urlquery") should only occur in
Identifier nodes, and never in Field or Chain nodes, since these are
global functions that return string values (see inline comments for more
details). Therefore, skip Chain and Field nodes when searching for
predefined escapers in template pipelines.

Also, make a non-functional change two existing test cases to avoid
giving the impression that it is valid to reference a field of a
predefined escaper.

Fixes #20323

Change-Id: I34f722f443c778699fcdd575dc3e0fd1fd6f2eb3
Reviewed-on: https://go-review.googlesource.com/43296Reviewed-by: Samuel Tan <samueltan@google.com>
Reviewed-by: Mike Samuel <mikesamuel@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>