Move device/core files to device/base directory.

The files are being moved from device/core to device/base to follow
the de-facto standards in other directories (such as net/,
ui/, cc/, ...)

I also fixed the FOR_EACH_OBSERVER calls which are now prohibited
(git cl upload was complaining).

BUG=

Review-Url: https://codereview.chromium.org/2417043002
Cr-Commit-Position: refs/heads/master@{#425173}
diff --git a/chrome/browser/chrome_device_client.h b/chrome/browser/chrome_device_client.h
index aa6635efb..a4ed096d 100644
--- a/chrome/browser/chrome_device_client.h
+++ b/chrome/browser/chrome_device_client.h
@@ -5,7 +5,7 @@
 #ifndef CHROME_BROWSER_CHROME_DEVICE_CLIENT_H_
 #define CHROME_BROWSER_CHROME_DEVICE_CLIENT_H_
 
-#include "device/core/device_client.h"
+#include "device/base/device_client.h"
 
 #include <memory>