If a resource load is started as the CachedResourceLoader is going away, make sure we remove the CachedResource from the MemoryCache.

This was regressed in r149483.

BUG=257316
TEST=ResourceFetcherTest.StartLoadAfterFrameDetach

Review URL: https://chromiumcodereview.appspot.com/19597008

git-svn-id: svn://svn.chromium.org/blink/trunk@154852 bbb929c8-8fbe-4397-9dbb-9b2b20218538