[headless] Make browser service manifest overlay configurable.

Headless embedders need to specify mojo services that they
expose to the renderer. This patch lets them list the services
in HeadlessBrowser::Options and generates the corresponding
browser manifest overlay in HeadlessContentBrowserClient.

BUG=546953

Review-Url: https://codereview.chromium.org/2489673005
Cr-Commit-Position: refs/heads/master@{#432151}
9 files changed