Cleanup: Make Platform::crypto() required.

The feature can be turned off via a runtime flag, so not much to be gained by allowing NULL when it is on.

BUG=245025

Review URL: https://chromiumcodereview.appspot.com/23798002

git-svn-id: svn://svn.chromium.org/blink/trunk@156976 bbb929c8-8fbe-4397-9dbb-9b2b20218538