IPC fuzzer child process component

Fuzzer child process takes messages from a testcase file specified by --ipc-fuzzer-testcase and sends them across IPC.

Renderer process is replaced by the fuzzer process using --renderer-cmd-prefix, which is only supported under POSIX.

BUG=260848

Review URL: https://codereview.chromium.org/18254010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237795 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237795