See bug 47874 for some background ... there are places in the code where using MI would make the code cleaner, like webkitpy/layout_tests/port/{google_chrome,chromium_gpu}.py , where the classes naturally want some sort of diamond and/or mixin behavior, and we resort to unpleasantness in its absence.
We use multiple inheritance in a bunch of places in webkitpy, mostly int the Mixin pattern.
Actually, also to inherit from "pure virtual" interfaces.
I'm fine with pure virtual interfaces and to a lesser degree, mixin. The proper way to refactor the google_chrome and chromium_gpu ports would be to have delegate classes for specific functionality of a port. For example, having a delegate that provides a few path functions (e.g., baseline search path and path to expectations) and have it be a member variable of Port. This could also be done via mixins, but I think the delegate pattern is more widely used in WebKit code.
To clarify, I think what we want to avoid is a GPU port and a Linux port where we inherit from both to make a Linux GPU port. That would be confusing because you don't know what functions are provided by which base class.
I think I will consider this closed as "yes, with caution". Anyone please reopen or comment if you disagree :)