[everybody CC'ed is FYI; this is a summary of an offline discussion]

Fresh STHs are provided to Chrome clients via the component updater - particularly via the STHSetComponetInstallerTraits (https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/component_updater/sth_set_component_installer.h).
STHs are needed by the TreeStateTracker (components/certificate_transparency) for checking inclusion proofs against.

The difficulty lies in the wiring of the two:
* The STHSetComponentInstallerTraits is created in chrome_browser_main, *after* the IOThread object is created (but not, it should be noted, on the IO thread).
* The various TreeStateTrackers are created in the IOThread/ProfileIOData, some before the STHSetComponentInstallerTraits is created, some after.

The question boils down to how to share an STHDistributor instance between the IOThread and the STHSetComponentInstallerTraits.

After discussion with sorin@ and rsleevi@ we came to the following conclusion:
- The STHDistributor should be accessible via a global defined somewhere under chrome/browser/net.

Alternatives considered and disqualified:
- Threading the STHDistributor to the IOThread from chrome_browser_main via BrowserProcessImpl (the STHDistributor is passed from chrome_browser_main -> BrowserProcessImpl that posts a task to the IOTHread to set  the STHDistributor). Disqualified because it creates a tight coupling throughout the code between the component updater and net stuff.

- Creating the STHDistributor in RegisterSTHSetComponent and setting it by post via g_browser_process (g_browser_process->io_thread()). Disqualified because access to g_browser_process is strongly discouraged, particularly so if it's being introduced in a new call site that did not previously depend on it.

- Static on the SSLConfigService, similar to how the CRLSet works: Approach is conceptually OK, but the static should live under chrome/browser/net as not all Chromium embedders may choose to operate this way.

More detailed notes from the discussion about wiring:
- An agreed-upon way to share the STHDistributor object seems to be through a global singleton which would be defined in chrome/browser/net (in a new file).
- The STHSetComponentInstallerTraits will get an instance of the STHDistributor that will be obtained from the global object in RegisterSTHSetComponentInstaller.
- The IOThread will also use this global singleton when it has to register/unregister an STH Observer (which server the purpose of abstracting the presence of a singleton from the Profile implementation).