Fix http/tests/images/drag-image-to-desktop.html

PHP 5.4.0 and newer implement hex2bin natively. Since PHP versions
aren't uniform across platforms, just prefix it and hope it doesn't
collide with another function in the future.

BUG=none

Review URL: https://codereview.chromium.org/724863003

git-svn-id: svn://svn.chromium.org/blink/trunk@185322 bbb929c8-8fbe-4397-9dbb-9b2b20218538