AW: fix variable name capitalization

No change to logic. While working on another change, I noticed a
presubmit warning about a variable name and class name.

AwSupervisedUserTest.java:117:34: Abbreviation in name
'mIFrameLoadedListener' must contain no more than '1' consecutive
capital letters.
(also saw the same warning for the class 'IFrameLoadedListener')

Test: git cl presubmit --upload
Change-Id: Ic2cc3cf255fe5b6f482b6a69cccb8d7583ee2346
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5936091
Commit-Queue: Peter Pakkenberg <pbirk@chromium.org>
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Peter Pakkenberg <pbirk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1369250}