Simplify functions in field_filler.cc

This CL aims at simplifying the logic in field_filler.cc by:
-Using modern syntax
-Reducing the degree of nesting
-Reducing code duplication

Bug: 1495294
Change-Id: Ic5ea6e8f7ca04e5115b0a04ab190a5825b008d73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4980126
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Jihad Hanna <jihadghanna@google.com>
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1216867}