Fix User->image() without ImageSkia representation after login

If the user chose an external avatar image (a downloaded file or a webcam
shot), the avatar image would remain invalid until the external image has
been loaded and decoded. This led to a race on login where the image may
be accessed before the load has completed. The CL fixes this by
substituting a stub image while the load is in progress.

BUG=331365
TEST=Manual

Review URL: https://codereview.chromium.org/136763003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244536 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244536