Re-attempt to fix the calendar widget.  Previous attempts
to land this patch have failed with memory corruption
errors on CrOS which only appear on the waterfall.  sky@
has pointed out a potential null pointer dereference, so
this is an attempt to see if adding a null check fixes the
issue.

Unfortunately we know of no way to test this without
actually committing the patch (since the issue does not
seem to reproduce on trybots), so sheriffs take note that
if this CL shows up on the blame list for a buildbot
failure in CrOS content tests, please revert it.

BUG=176221
TBR=sky

Review URL: https://codereview.chromium.org/40263005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231069 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231069