Revert 142482 - Add untrusted NaCl build for PPAPI proxy.

Revert 142482 - Add untrusted NaCl build for PPAPI proxy.
This patch refactors ppapi_shared.gypi and ppapi_proxy.gypi into proper includes, adds ppapi_shared_untrusted and ppapi_proxy_untrusted .gyp files, and integrates them into the nacl_irt build (ppapi/native_client/native_client.gyp). In order to build without link errors, it includes our plugin side initialization of PluginDispatcher, and a PpapiPluginMain definition. When the 'build_ppapi_ipc_proxy_untrusted' gyp flag is set to '1', this will build a working NaCl IRT using the Chrome IPC proxy.
BUG=116317
TEST=compiles, runs HelloWorld and GetURL SDK examples.

Review URL: https://chromiumcodereview.appspot.com/10546140

TBR=bbudge@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=142494