Remove assumption that backing type is a string.

Noticed recently this method chops off values without checking to make
sure they are what it expected. Moving the comment to an actual
assertion so that if this ever changes this is caught, or if this
assumption isn't valid, the fuzzer will catch it.

Bug: N/A
Test: N/A
Change-Id: I8e7ea6cc14d49e8685a4eac47c92b64028712ddc
1 file changed