bug#49629: 27.2; electric-pair-mode doesn't work for angle brackets in HTML file

[Allen Li]

To reproduce:

1. emacs -Q
2. C-x C-f /tmp/tmp.html RET
3. M-x electric-pair-local-mode RET
4. < >

Expected:

Buffer contains <>

Actual:

Buffer contains <>>

By default, typing > should skip over the > inserted by
electric-pair-mode when the first < is type (but it doesn't).

I tried stepping through with Edebug, but the bug disappears when using
Edebug.  The difference seems to be in what is returned by
electric-pair--balance-info, but that function is big and unfamiliar to
me.

In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.27, cairo 
version 1.17.4)
 of 2021-03-26 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12012000
System Description: Arch Linux