Don't use the empty string as an url in MemoryCacheTest.

MemoryCacheTest relies on KURL doing something useful with an empty string
that results in a non-empty string that can be used as hash key for
the MemoryCache. Better to not do that because KURL is being changed.

BUG=

Review URL: https://codereview.chromium.org/346263003

git-svn-id: svn://svn.chromium.org/blink/trunk@176773 bbb929c8-8fbe-4397-9dbb-9b2b20218538