Revert 156819 "ResourceFetcher declines blocked image read inste..."

Sorry for the trouble. It turns out that this change introduces a
regression. Images blocking isn't permanent, it can be changed
dynamically, which must result in loading of previously blocked
images without reloading the page. This patch breaks this
behavour.

> 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

TBR=kouhei@chromium.org

Review URL: https://codereview.chromium.org/23711002

git-svn-id: svn://svn.chromium.org/blink/trunk@156851 bbb929c8-8fbe-4397-9dbb-9b2b20218538