Pulled out Callback code into base/callback.h.  This is the first step towards redoing the Callback interfaces.

Added and removed includes as needed.

BUG=35223
TEST=trybots

Review URL: http://codereview.chromium.org/646061

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@39419 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/base/base.gypi b/base/base.gypi
index 9cd768b..66094b9d 100644
--- a/base/base.gypi
+++ b/base/base.gypi
@@ -40,6 +40,7 @@
           'basictypes.h',
           'bits.h',
           'bzip2_error_handler.cc',
+          'callback.h',
           'cancellation_flag.h',
           'cancellation_flag.cc',
           'chrome_application_mac.h',