Remove SSLKeyGenerator.{cpp,h}.

They are very thin wrappers of blink::Platform. We may use blink::Platform
directly.

TEST=none; no behavior changes.
BUG=

Review URL: https://codereview.chromium.org/585793002

git-svn-id: svn://svn.chromium.org/blink/trunk@182304 bbb929c8-8fbe-4397-9dbb-9b2b20218538