Make DevToolsManager a singleton, remove it from ContentBrowserClient

Make DevToolsManager a singleton, remove it from ContentBrowserClient.

DevToolsManager should be created and destroyed inside content implementation, not by the embedder.

BUG=104625
TEST=Exising tests

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=111106

[yurys, 2011-11-21 09:41:46]

Since DevToolsManager now listens to NOTIFICATION_MAIN_LOOP_EXITING to delete
itself, it will leak in devtools_manager_unittest.cc I see several possibilities
for fixing this:
1. Make DevToolsManager a non-leaky singleton that would be deleted at exit.
2. As DevToolsManager::GetInstance is expected to be accessed from the UI thread
only we can use DevToolsManager* g_instance which would be reset to NULL when
DevToolsManager is destroyed. This way we could delete DevToolsManager
explicitly in the unittests that don't run the browser main loop.

[yurys, 2011-11-21 15:57:55]

As suggested in
http://codereview.chromium.org/8549022/diff/4001/content/browser/browser_main...
I made DevToolsManager a non-leaky singleton(option 1 described above) and
reverted all changes to the notification service.

[pfeldman, 2011-11-21 16:01:57]

lgtm

[jam, 2011-11-21 18:30:58]

lgtm, thanks