Schedule GC of document resources when loading data URLs

If a page loads a large number of data URLs, without any intervening loads
of other resources, ResourceFetcher::m_documentResources will hold
references to the data URL Resources, without attempting to remove ("GC")
them.
Scheduling a GC of Resources after having created a data URI Resource to
prevent accumulation.

BUG=309543

Review URL: https://codereview.chromium.org/133013006

git-svn-id: svn://svn.chromium.org/blink/trunk@164879 bbb929c8-8fbe-4397-9dbb-9b2b20218538