Always erase CBF_SMBITMAP from the clipboard object map for async writes

This can never be a pointer to a valid object, since this CBF_SMBITMAP
is only used for synchronous writes to the clipboard. This accidentally
regressed in http://crrev.com/123088.

BUG=319125
R=jorgelo@chromium.org
TBR=creis@chromium.org

Review URL: https://codereview.chromium.org/72483002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235085 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235085