Refactor ElementIdInfo

ResponsiveElementInfo.observed_identifier is only present when `ElementIdInfo` is used for overflow_id. Thus, it makes much more sense to put `observed_element` inside `ElementIdInfo`. It makes the code easier to reason about (at least to me :)).

I also added an explicit constructor to `ElementIdInfo` so that the compiler catches all cases in which `ElementIdInfo` is instantiated so that we correct the code to use the correct constructor.

Bug: 359863838
Change-Id: I17a2a63ac97331ab74c27f4e3cdc555300ca121d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6020323
Reviewed-by: David Pennington <dpenning@chromium.org>
Commit-Queue: André Bianchessi <abianchessi@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1385735}