Chromecast: moves all service startup code to PreMainMessageLoopRun.
On Android, CastService::Start is never called (!). This is because
Android uses a Java-based UI message loop after
BrowserMainRunner::Initialize is completed, and BrowserMainRunner:Run is
never called (nor should it be).
See ChromeBrowserMainParts for similar handling of the Android case.
R=lcwu@chromium.org,byungchul@chromium.org
BUG=internal b/
18934906
Review URL: https://codereview.chromium.org/
953283002
Cr-Commit-Position: refs/heads/master@{#318136}