Clear ResourceFetcher::m_validatedURLs if too many entries are added

This is to avoid memory bloat due to m_validatedURLs.

BUG=52411

Review URL: https://codereview.chromium.org/723423002

git-svn-id: svn://svn.chromium.org/blink/trunk@185369 bbb929c8-8fbe-4397-9dbb-9b2b20218538