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Before this CL, FindFormControlElementsByUniqueRendererId did many
linear searches over its vector parameter |form_control_renderer_ids|,
and accordingly had a DCHECK that the vector doesn't contain more than
10 elements.
This CL changes the implementation to build and use a map instead for
the lookups, thus avoiding the linear searches, and making
FindFormControlElementsByUniqueRendererId appropriate for larger
inputs.

Bug: 1004777
Change-Id: I0699d87de7202f42269c3504ecf08356ee880601
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899455Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712552}