ResourceFetcher declines blocked image read instead of deferring.

Before this patch, the <img> elements having blocked URL as their src leaked, as its loader never completes.

This patch changes ResourceFetcher::requestImage()'s behavior to decline blocked URL image load instead of deferring it. Previously, they were set as DeferredByClient, but no code path exist to actually load those "deferred" resources when they are unblocked.

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/22918024

git-svn-id: svn://svn.chromium.org/blink/trunk@156819 bbb929c8-8fbe-4397-9dbb-9b2b20218538