DevTools: Extract target discovery and manipulation from DevToolsHttpHandlerImpl

Target discovery has moved into DevToolsHttpHandlerDelegate. 

The new DevToolsTarget interface is introduced to encapsulate
access to target details (id, title, url etc) and manipulation 
(activate, close, inspect).

It is now possible to have an implementation of DevToolsTarget
tied to a tab id instead of a RenderViewHost instance (which is the
prerequisite for fixing the referenced bug).

BUG=272174

Review URL: https://codereview.chromium.org/24995003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226771 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226771