[MD settings] cast elements to cr_dialog
This CL adds type checking for dialog elements
found by this.$.dialog. There is no functional
change aside from the type checking.
BUG=
393873
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation
Committed:
https://crrev.com/016c0bd1748c39a60bc4561269c5ac9c52811f4e
Cr-Commit-Position: refs/heads/master@{#403347}