[DevTools] Refactored dispatcher, to simplify adding new dispatchers.

BUG=309879

Review URL: https://codereview.chromium.org/186753003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255087 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255087