Fix mojo_shell DataPipePeek tests

If a Context hadn't been created by the time the DataPipePeek.* mojo_shell_tests ran, they'd fail:

#3 0x7fdfef372414 __GI___pthread_mutex_lock
#4 0x0000005baa3b base::internal::LockImpl::Lock()
#5 0x000000459f35 base::Lock::Acquire()
#6 0x00000045c673 base::AutoLock::AutoLock()
#7 0x00000076d313 mojo::system::Core::CreateDataPipe()
#8 0x000000b97d51 MojoCreateDataPipe

Context::Init() creates a mojo::embedder::PlatformSupport() singleton which the tests depend on.

Added Context::SetupEmbedder() which ensures that the singleton has been constructed.

BUG=
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/726903004