Make platform depend on openmax_dl.

This is required for upcoming openmax_dl changes here:
https://codereview.chromium.org/626183003

but I believe should be done in any case.

BUG=415393

Review URL: https://codereview.chromium.org/635443002

git-svn-id: svn://svn.chromium.org/blink/trunk@183311 bbb929c8-8fbe-4397-9dbb-9b2b20218538