Use validating IPC enum macros in drag_traits.h

This is part of a long-running background task to remove the remaining
uses of the unchecked IPC_ENUM_TRAITS() macro.

BUG=246708

Review URL: https://codereview.chromium.org/149443009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249754 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249754
Add 64-bit support to browser blacklisting

BUG=

Review URL: https://codereview.chromium.org/101203010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249755 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249755