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Currently this is either a list or a string. However, this is causing some problems because templates can't add to the invoker's visibility list without knowing if the original is a string or a list.

In an effort to make this consistent, I'm converting all visibiltiy to be lists, and will remove support for strings in a future build.

This exempts cld from header checking since it was confusing GN's header checker. It adds a ppapi header target as well that will be used by libyuv (that requires a roll).

TBR=scottmg

Review URL: https://codereview.chromium.org/544423002

Cr-Commit-Position: refs/heads/master@{#293638}