Split ExtensionSystem interface from ExtensionSystemImpl implementation, part 1.

This moves ExtensionSystem to extensions/browser.
It also moves setting the channel for Features to ChromeExtensionsBrowserClient.
It also removes the duplicate ExtensionSystem::GetForBrowserContext.

BUG=337707
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Review URL: https://codereview.chromium.org/147923005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247788 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/browser/extensions/extension_service.cc b/chrome/browser/extensions/extension_service.cc
index d404fbf9..f7f11403 100644
--- a/chrome/browser/extensions/extension_service.cc
+++ b/chrome/browser/extensions/extension_service.cc
@@ -40,7 +40,6 @@
 #include "chrome/browser/extensions/extension_install_ui.h"
 #include "chrome/browser/extensions/extension_special_storage_policy.h"
 #include "chrome/browser/extensions/extension_sync_service.h"
-#include "chrome/browser/extensions/extension_system.h"
 #include "chrome/browser/extensions/extension_util.h"
 #include "chrome/browser/extensions/external_install_ui.h"
 #include "chrome/browser/extensions/external_provider_impl.h"
@@ -77,6 +76,7 @@
 #include "extensions/browser/app_sorting.h"
 #include "extensions/browser/event_router.h"
 #include "extensions/browser/extension_registry.h"
+#include "extensions/browser/extension_system.h"
 #include "extensions/browser/extensions_browser_client.h"
 #include "extensions/browser/external_provider_interface.h"
 #include "extensions/browser/management_policy.h"