media: Support hosting mojo CDM in a standalone service

Currently when mojo CDM is enabled it is hosted in the MediaService
running in the process specified by "mojo_media_host". However, on
some platforms we need to run mojo CDM and other mojo media services in
different processes. For example, on desktop platforms, we want to run
mojo video decoder in the GPU process, but run the mojo CDM in the
utility process.

This CL adds a new build flag "enable_standalone_cdm_service". When
enabled, the mojo CDM service will be hosted in a standalone "cdm"
service running in the utility process. All other mojo media services
will sill be hosted in the "media" servie running in the process
specified by "mojo_media_host".

BUG=664364
TEST=Encrypted media browser tests using mojo CDM is still working.

Change-Id: I95be6e05adc9ebcff966b26958ef1d7becdfb487
Reviewed-on: https://chromium-review.googlesource.com/567172
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486947}
void ServiceManagerConnectionImpl::AddEmbeddedService(
    const std::string& name,
    const service_manager::EmbeddedServiceInfo& info) {
  context_->AddEmbeddedService(name, info);
}

void ServiceManagerConnectionImpl::AddEmbeddedService(
    const std::string& name,
    const service_manager::EmbeddedServiceInfo& info) {
  context_->AddEmbeddedService(name, info);
}
