Removes the global build flag and #define and converts to a buildflag header in
ppapi/features.

A new target "//third_party/widevine/cdm:headers" was added since
widevine_cdm_version.h was unreferenced in the build. Given that this file now
requires //ppapi/features we really need "gn check" to work on it. I made the
existing users of ":version_h" reference this new target which encompasses the
public headers of the widevine CDM.

TBR=jrummell@chromium.org (original reviewer)
TBR=jschuh@chromium.org (render_messages.h owners)
TBR=ddorwin@chromium.org (widevine owners)

Reland of http://crrev.com/2411433002 with official build fix.

Review-Url: https://codereview.chromium.org/2409423002
Cr-Commit-Position: refs/heads/master@{#424687}