Simplify inheritance around FormGroup::GetInfo.

Preexisting state: there were 3 methods 2xGetInfo, GetInfoImpl,
GetRawInfo. In general they were calling one another in set order but
there were quirks: GetRawInfo was pure virtual, GetInfoImpl had default
implementation that was overridden sometimes and GetInfo was never
overridden. In addition, GetInfo didn't do anything other than calling
GetInfoImpl. This made the excecution flow hard to understand when
reading.

With this CL: both GetInfo methods were made pure virtual, GetInfoImpl
was removed altogether. Now the code should be easier to read as a
situation when implementation is sometimes in base class and sometimes
in derived was eliminated, perhaps other refactors are possible. A
downside is that we now have 2 boilerplate implementations of GetInfo
everywhere (until we get rid of AutofillType version altogether) but IMO
it's acceptable tradeoff.

Low-Coverage-Reason: LARGE_SCALE_REFACTOR
Bug: 389018075
Change-Id: Ib3860dfd5c3fff6914a1936d555382f67f3e1690
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6177740
Commit-Queue: Piotr Domaradzki <doland@google.com>
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Reviewed-by: Karol Sygiet <sygiet@google.com>
Cr-Commit-Position: refs/heads/main@{#1408953}