[perf] Never delete dict while iterating.

This throws a RuntimeError and should be avoided. The new
implementation makes a copy only of keys that are not 'undefined'
or False-y.

Change-Id: I6fa52f9ff743af0b4c439c8b7a0bb5e9fceed470
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3947493
Commit-Queue: Rohan Pavone <rohpavone@chromium.org>
Reviewed-by: Wenbin Zhang <wenbinzhang@google.com>
Cr-Commit-Position: refs/heads/main@{#1058204}