Validate vector lengths in ImageHostMsg_DidDownloadImage

BUG=333038

Review URL: https://codereview.chromium.org/133123006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245618 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245618