Move chromiumcontent.gyp into its own directory

This will give us a place to put more files to be copied into src/.
Teach gyp to link libchromiumcontent as C++

As of https://codereview.chromium.org/23475025, gyp will only link
modules as C++ if they contain at least one C++ source file. So now we
include an empty C++ source file in the target.