WebImageCache: only clear if the underlying instance is available.

The clear() API is exposed as static and can be called after the
underlying instance has been shutdown (or not created).

BUG=286402

Review URL: https://chromiumcodereview.appspot.com/23597021

git-svn-id: svn://svn.chromium.org/blink/trunk@157469 bbb929c8-8fbe-4397-9dbb-9b2b20218538