Inlines the sources from ppapi_sources.gypi into the respective BUILD.gn files.
Previously the cpp sources were compiled into two targets, one in //ppapi:ppapi_cpp_sources (referenced only from nacl), and one in //ppapi/cpp (referenced only from trusted). This patch merges them into the cpp directory, which now morphs as required for the build type.
R=bbudge
TBR=bradnelson (chrome/test/data/nacl)
Review-Url: https://codereview.chromium.org/2289983002
Cr-Commit-Position: refs/heads/master@{#415183}
9d6a868b4d