WebImageCache: only clear if the underlying instance is available.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@157469 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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