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Simplified ObserverListBase.

Made ObserverListBase's iterator and const_iterator the same type. ObserverListBase::iterator doesn't allow to modify the collection anyway. There was no good reason for const_iterator and iterator to be different types. Making them the same type just makes the code simpler.
Inlined ObserverListBase methods.
Removed unnecessary friend declarations.
Removed old MSVC workarounds.

In the end: less code to do exactly the same thing.

Change-Id: I0448a8a52f904635a2e30d094bcc2b48780e4d64
Reviewed-on: https://chromium-review.googlesource.com/773758
Commit-Queue: François Degros <fdegros@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517775}