Refactor GlobalShortcutListener.

1. Move method [Start/Stop]Listening from public to private.
2. The change of relationship between accelerator and observer: from
"one-to-many" to "one to one".
3. Better name: use suffix "Impl" for the platform-specified implementation
methods: |RegisterAcceleratorImpl| and |UnregisterAcceleratorImpl|.

BUG=329616, 302437

Review URL: https://codereview.chromium.org/109413003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242995 0039d316-1c4b-4281-b951-d872f2087c98
Switch RenderProcessHostTest to observe the RenderProcessHost rather than use notifications.

BUG=170921
TEST=no change

Review URL: https://codereview.chromium.org/124203002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242996 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242996