MemoryCache: Remove ENABLE(ASSERT) checks for readability

There is no behavioral change.

BUG=n/a
TEST=should pass all existing tests

Review URL: https://codereview.chromium.org/660723002

git-svn-id: svn://svn.chromium.org/blink/trunk@183794 bbb929c8-8fbe-4397-9dbb-9b2b20218538