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This removes some special-case code used to register and load
gtbp and o1d PPAPI plugins on Linux and ChromeOS. It also removes
infrastructure used to launch these plugins.

TEST=Run PDF (out-of-process) and NaCl (in-process) plugins
BUG=392002

Review URL: https://codereview.chromium.org/374773002

Cr-Commit-Position: refs/heads/master@{#324131}